Line a large baking sheet with parchment paper; set aside.: The sheet will feel cool to the touch and the parchment should lie flat without wrinkles, which prevents uneven baking. Using parchment helps cookies release cleanly and promotes even browning on the bottom, so I always line the pan even if I’m in a hurry. If the parchment curls, press it down with a second sheet or a light pan to flatten it out. A common mistake is skipping parchment and placing dough on an unlined pan, which can cause sticking and uneven browning.
Preheat oven to 375 degrees (F). In a medium sized bowl add the flour, cornstarch, cocoa powder, salt, and baking soda; whisk well to combine and set aside. In a large bowl using a handheld electric beater, or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter and vanilla together on medium-speed until smooth and creamy. Add in both sugars and beat until light and fluffy; about 2 minutes. Add in eggs, one at a time, beating well after each addition. Gradually add in the dry ingredients, mixing until just combined. Using a rubber spatula, fold in chocolate chips and butterscotch chips. Drop 2 tablespoon sized balls of dough onto the prepared sheet, leaving 1" between each cookie for spreading. Bake for 9-12 minutes (depending on your oven), or until the edges are set and centers look just slightly underdone. Cool for 5 minutes on the cookie sheet before transferring to a wire rack to cool completely.: As the oven warms you will notice a faint dry, warm smell and the internal fan may cycle. Hitting the correct temperature ensures the cookies set quickly at the edges while keeping centers tender. I wait until the oven signals ready because baking into a too cool oven causes excess spread. One trap is assuming a preheat is quick; modern ovens can take longer, so use an oven thermometer if you want precision.
In a medium sized bowl add the flour cornstarch cocoa powder salt and baking soda whisk well to combine and set aside: As you whisk, the cocoa will dust the bowl and the mixture should look uniformly brown with no streaks of flour. This ensures even flavor and leavening in every bite. Whisking aerates the dry mix slightly, which helps incorporation without over mixing later. Avoid over stirring the dry mix for too long, as it can scatter and create a mess, but be thorough enough that there are no lumps.
In a large bowl beat the butter and vanilla together on medium speed until smooth and creamy: You will hear the mixer hum and see the butter soften into a pale, glossy paste, smelling rich and dairy forward. This step introduces air which builds structure and affects final texture, so beat until light and smooth. Under creaming leaves dense pockets, while over creaming can cause too much spread; aim for a creamy consistency that ribbons slightly when lifted.
Add in both sugars and beat until light and fluffy about 2 minutes: The mixture will lighten in color and take on a pillowy texture, with a faint granulated sound as the sugars dissolve into the butter . This trapped air is crucial for lift and texture. If the mix still looks gritty, scrape the bowl and beat a few more seconds. A common error is rushing this step, which reduces volume and yields flatter cookies.
Add in eggs one at a time beating well after each addition: As each egg is incorporated the batter will smooth and become slightly looser, with a glossy sheen. Adding eggs slowly creates an emulsion that holds ingredients together, preventing curdling. If you add eggs too quickly the batter can split; if separation happens, slow mixing and a small splash of flour can help bring it back.
Gradually add in the dry ingredients mixing until just combined: You will notice the batter thicken and darken as the dry mixture folds in, taking on a slightly stiffer scoopable texture. Mixing just until no streaks remain preserves tenderness; over mixing develops gluten and makes cookies tough. Use a low speed or fold by hand, and stop as soon as the flour is incorporated to avoid a dense result.
Using a rubber spatula fold in chocolate chips and butterscotch chips: Folding keeps the chips intact and evenly distributed, and you should see glossy chips embedded across the dough. The batter will feel chunky and studded, and the chips will shine against the dark dough. Avoid over folding, which can deflate the batter, and be sure to reach the bottom of the bowl so chips do not clump in one area.
Drop 2 tablespoon sized balls of dough onto the prepared sheet leaving 1 inch between each cookie for spreading: The dough scooped will be dense and hold shape, with chips peeking through. Spacing is crucial because the cookies will spread; giving them room prevents merging. If the dough is too soft to shape, refrigerate briefly. A frequent mistake is placing scoops too close, resulting in a single merged sheet of cookie.
Bake for 9 to 12 minutes or until the edges are set and centers look just slightly underdone: As they bake you will smell chocolate and toffee notes, and the edges will take on a matte look while the centers remain slightly glossy. Pulling them at the right moment yields a soft center that firms as they cool. Overbaking leads to a dry interior, so err on the sooner side if you prefer chewiness. If the cookies brown too fast, lower the oven by 10 to 15 degrees next time.
Cool for 5 minutes on the cookie sheet before transferring to a wire rack to cool completely: During this brief rest the cookies continue to set and the centers reach the perfect texture, firming slightly as steam escapes. The bottoms should feel set but not hard when you move them, and the chips will solidify but remain tender. Moving them too early can cause breakage, while leaving them too long on a hot sheet can make bottoms soggy, so aim for that five minute window.