Preheat the oven to 350°F. Grease a 9" x 13" pan and set aside.: You will notice a warm kitchen scent as the oven comes up to temperature, which signals stable baking conditions. Greasing the pan prevents sticking and helps the edges crisp slightly; I use a light coat of butter and a parchment sling for easy removal. A common mistake is not letting the oven fully preheat, which can lead to uneven rise and a gummy center. Tap the pan gently on the counter to settle any air pockets before pouring batter so the surface bakes evenly.
In a large saucepan, melt the butter over low heat. Add the sugar and stir to combine. Return the mixture to the heat, just until it’s hot, but not bubbling. It will become shiny as you stir. Remove from heat and stir in the cocoa, baking powder, salt, and vanilla extract.: As you melt the butter , you will smell a rich, buttery aroma that carries into the cocoa when combined. The sugar dissolves into the warm fat, creating a glossy base which helps form that desirable crackly top. When you fold in the cocoa and leavening, the batter darkens and thickens. If you allow the mixture to bubble, the sugar may caramelize and alter the texture, so keep the heat low. Stirring until glossy ensures the ingredients are incorporated, preventing dry spots of cocoa in the baked brownies.
Add the eggs, stirring until smooth. Add the flour and stir until just combined.: At this point the batter will smell rich and chocolate forward, with a shine from the butter and sugar. The eggs should be at room temperature so they blend without seizing; when they are fully mixed in, the batter becomes silky. Adding the flour last minimizes gluten development. You will feel the batter thicken but it should remain slightly fluid. Avoid overmixing here, because too much stirring causes the brownies to be cakier instead of fudgy. If you see streaks of flour, fold gently until they disappear.
Spoon half of the batter into prepared pan. Place the peppermint patties on top of the batter. Pour the remaining batter over the peppermint patties and gently smooth. Tap the pan on the counter to even the batter out.: When you spoon the first layer, you will see a glossy base about halfway up the pan. Lay the Peppermint Patties in an even grid so each portion gets a piece. Pouring the remaining batter over them creates a chocolate blanket that locks the patties in place. Tapping the pan removes air pockets and helps the batter settle around each candy. A common error is crowding the patties too tightly, which can cause large gaps in the final squares; space them evenly for consistent slices.
Bake the brownies for 35 minutes, until a toothpick inserted into the center comes out clean, or with just a few moist crumbs clinging to it. The brownies should feel set on the edges, and the center should look very moist, but not uncooked. Remove them from the oven and cool completely.: During baking you will notice the aroma intensify as cocoa and butter mingle in the heat. The glossy top may develop small cracks, a sign of a shiny crust. Edges will pull away slightly from the pan, feeling springy to the touch, while the middle should still jiggle a bit before cooling. Overbaking dries them out, so start checking near the end of the time. If a toothpick comes out wet with batter, they need more time; if it is completely dry, they may be past prime; aim for a few moist crumbs for ideal fudginess.
To make the ganache, add the chocolate to a large bowl. Heat the cream in a small saucepan over medium heat just until it bubbles around the edges. Remove the cream from the heat and pour it over the chocolate. Let the mixture stand for 30 seconds, then stir until the chocolate melts and a smooth ganache comes together, about 1-2 minutes. Pour it over the top of the brownies, then cover with the chopped peppermint patties. Let the brownies sit for 30 minutes before cutting. Cut, serve, and enjoy!: The moment you pour warm cream over the semi-sweet chocolate chips you will smell the rich chocolate envelope the cream, and within seconds the chocolate softens into glossy ganache. Letting it stand allows heat to gently soften the chips, so stirring creates an ultra smooth texture. Pouring it over cooled brownies gives a shiny finish and helps the ganache set without sinking. Scatter the chopped Peppermint Patties while the ganache is still wet so they adhere. A common slip is pouring ganache on warm brownies, which causes it to sink in; always ensure the bake is cool to touch for a neat top. After sitting, the ganache firms slightly but remains creamy, and the candies provide a final peppermint crunch that sings against the fudgy base.