Preheat the oven to 350℉. Spray a 9X9 inch baking pan with baking spray. Line the bottom and 2 sides of the pan with parchment paper and spray again.
In a medium bowl, sift the flour, cocoa powder, cornstarch, and salt. Set aside.
1 cup All-purpose flour, 1 cup Unsweetened cocoa powder, 1 TBSP Cornstarch, 1 tsp Salt
Using a mixer, beat the sugar, powdered sugar, eggs, and egg yolk on high speed for 2 minutes. It will become pale in color.
1 1/2 cups White granulated sugar, 1/2 cup Powdered sugar, 3 Large eggs, 1 Egg yolk
Add in the melted butter, oil, and vanilla. Mix on low speed until combined. Add in the dry ingredients and mix on low until just combined.
1/2 cup Unsalted butter, 1/2 cup Oil, 1 tsp Pure vanilla extract
Take off the mixer and use a baking rubber spatula to mix the batter. Add in the chocolate chips and peppermint chips and mix until combined.
1 cup Semi-sweet chocolate chips, 1 cup Andes peppermint chips
Pour the batter into the pan. Spread evenly.
Bake for 33-40 minutes. Bake until the edges are set but the middle is slightly underdone.
Place the hot pan on a cooling rack and cool completely before cutting. The brownies will continue to bake as it cools. Then, cut and serve.