Preheat oven to 325°F. Spray an 8X8 pan with baking non-stick spray. Line the bottom and 2 sides of the pan with parchment paper. Spray again.
In a mixing bowl, sift the flour, cocoa powder, cornstarch and salt. Using a mixer with a whisk attachment, combine the sugar, eggs, and egg yolk. Beat on high for 5 minutes.
1 ⅓ cup White granulated sugar, 2 Large eggs, ½ cup All-purpose flour, ¾ cup Dutch process cocoa powder, 1 tablespoon Cornstarch, ½ teaspoon Salt, 1 Egg yolk
While the eggs are mixing, melt the butter. Let it slightly cool. Add in melted butter, oil, and vanilla. Mix on low. Add in the dry ingredients. Mix on low until combined. Scrape the bowl. Mix again.
5 tablespoon Unsalted butter, ⅓ cup Oil, 1 teaspoon Pure vanilla extract
Take the brownie batter off the mixer. Mix the batter with a rubber spatula to make sure it is mixed. Fold in the chocolate chips and chopped peanut butter cups.
½ cup Semi-sweet chocolate chips, ¾ cup Reese's peanut butter cups
Pour batter into the pan. Bake for 45-55 minutes. The brownies need to be slightly underdone in the middle. The edges will be set and no more jiggle in the middle of the brownies. Top with more chopped peanut butter cups. This is optional. Let the brownies cool completely. Remove from pan, cut and serve.