Preheat oven to 350 degrees F. Grease an 8×8 baking pan with butter or nonstick spray.: The oven warming fills the kitchen with an anticipatory warmth and ensures even baking right from the start, which is crucial for a uniformly set brownie . You should hear a faint hum as the heating cycles kick in and see the temperature stabilize before the pan goes in. Greasing the pan prevents sticking and allows for clean release, so use a paper towel to spread the melted butter thinly across corners. A common mistake is putting the batter into an oven that is still cold, which can result in uneven rise and a gummy center, so wait until the oven reads the correct temperature. Also, make sure the pan is the correct size, because a too large pan will produce thin, overbaked layers while a too small pan will give underbaked centers.
In a large bowl, combine melted butter with sugar and vanilla extract, whisking until combined and almost smooth. Whisk in each egg one at a time, stirring until a smooth batter forms. Add in flour, cocoa powder, baking soda and salt, stirring until combined. Fold in chocolate chips. Spread batter in the 8×8 baking dish and bake for 25-30 minutes until set. Remove and let cool.: You will notice a glossy sheen as the granulated sugar starts to hydrate in the warm melted butter , and the vanilla scent will bloom into the mix, hinting at the layered flavors to come. Whisking at this stage creates a cohesive base that traps tiny air pockets, contributing to a tender crumb. Keep the whisk moving so the mixture becomes uniform and not grainy. A frequent error is using butter that is too hot, which can scramble the eggs later; let it cool slightly before adding eggs in the next step. Also avoid over whisking which can incorporate too much air and change the texture.
While brownies are cooling, add powdered sugar, butter, milk, vegetable oil, vanilla and peppermint extracts to the bowl of an electric mixer. Beat until smooth and creamy. Pour over brownies and spread in an even layer with a spoon or spatula. Place in the refrigerator until set, about 30 minutes.: Adding each egg separately ensures it emulsifies fully into the butter and sugar, creating a silky batter that will carry the cocoa and flour smoothly. You can sense the batter becoming glossy and more fluid with each addition, and there should be no streaks of egg white or yolk. If you add the eggs too quickly or if the butter is still hot, the eggs may curdle leading to a lumpy batter, so be patient and mix until smooth before adding the next. The right consistency at this stage is slightly thick but pourable so it will spread evenly in the pan when folded with dry ingredients.
To make ganache, heat heavy cream until warm and pour it over chopped chocolate. Stir for a few minutes until totally combined, smooth and creamy. Spread over the top of the peppermint layer or cut brownies first and spoon over top. I love these refrigerated!: When you fold in the dry all-purpose flour and unsweetened cocoa powder , the batter darkens and takes on that classic brownie aroma, rich and slightly bitter, which balances the sugar. Stir just until there are no dry streaks, because overmixing develops gluten and yields a cakier texture instead of fudgy. You should stop as soon as the mixture looks uniform; the batter should cling to the spoon and move slowly when poured. A typical slip up is vigorously beating here, which introduces too much air and changes the chewy character you want.
Fold in chocolate chips.: Folding in the chocolate chips introduces pockets of melted bliss that punctuate the dense brownie matrix, and you should see the chips peeking through the batter like little promises of molten chocolate. The sound is subtle, a quiet scrape as the spatula moves, and the visual cue is a glossy, studded batter ready for the pan. Be gentle to keep the batter dense rather than aerated. A common oversight is adding too many mix ins which can cause sinking, so stick to the measured amount for even distribution.
Spread batter in the 8×8 baking dish and bake for 25 to 30 minutes until set. Remove and let cool.: As the batter bakes, the kitchen fills with a deep chocolate aroma and you may notice the edges pulling slightly from the pan, a sign of doneness. The center should look set but slightly soft, not liquid, and a toothpick inserted will come out with moist crumbs rather than raw batter. Once out of the oven, the brownies will continue to firm as they cool, so allow them to rest at room temperature until they are cool to the touch. Cutting into them too soon is a frequent mistake, producing smeared layers rather than clean squares, so patience is rewarded. If the top cracks a bit, that is normal and can add character once the peppermint and ganache are layered.
While brownies are cooling, add powdered sugar, butter, milk, vegetable oil, vanilla and peppermint extracts to the bowl of an electric mixer. Beat until smooth and creamy.: Mixing the peppermint layer combines the powdered sugar into a silk like paste when the softened butter and evaporated milk are incorporated, and the aroma of peppermint extract should be bright but not overpowering. The texture should be thick enough to hold shape when spread, yet pliable; if it seems dry, add the tiniest splash of milk, if too loose add a dusting more powdered sugar . Overbeating can make the mixture airy and less candy like, so stop when it’s glossy and homogeneous. A common issue is graininess from lumps in powdered sugar , so sift first if needed.
Pour over brownies and spread in an even layer with a spoon or spatula. Place in the refrigerator until set, about 30 minutes.: Spreading the peppermint layer should feel smooth, and the spoon will glide across the surface leaving a uniform, pale sheet that contrasts with the dark base. Refrigeration firms this layer into a candy like middle that will slice cleanly; you will see the layer turn slightly matte as it chills and becomes firm to the touch. Avoid dipping the spatula back and forth repeatedly which can create ridges, instead make long, even strokes. Rushing the chill time is a common mishap, which can lead to a soft center that oozes when you add ganache.
To make ganache, heat heavy cream until warm and pour it over chopped chocolate.: When the warm heavy cream hits the chopped dark chocolate , you will notice steam and a soft, molten swirl as the chocolate begins to melt, and then the mixture becomes glossy and velvety as you stir. Let it sit for a minute before stirring to ensure the temperature equalizes for a smooth emulsion, then stir gently until there are no streaks left. Overheating the cream or stirring too vigorously while cold can cause a separated texture, so maintain a gentle rhythm. The resulting ganache should coat the back of a spoon in a shiny film.
Stir for a few minutes until totally combined, smooth and creamy. Spread over the top of the peppermint layer or cut brownies first and spoon over top. I love these refrigerated!: The finished ganache should be silky and spreadable, creating a reflective top that seals in the peppermint layer and adds a rich finish. If you spread it while a bit warm, it will level neatly and create a thin glossy cap; if you prefer thicker coverage, allow it to cool slightly before piling on. Watch for drips on the edges which you can smooth with the back of a spoon. Chilling again after ganache helps everything set and makes slicing neater. A frequent error is cutting too soon before the ganache firms, which can smear the top and ruin clean edges.