Preheat oven to 325°F. Spray a 8X8 pan with baking non-stick spray. Line the bottom and 2 sides of the pan with parchment paper and spray again. Set aside.
In a medium bowl, sift the flour and cocoa powder. Then, add in cornstarch and salt. Melt the butter in the microwave, then let it cool.
½ cup All-purpose flour, ¾ cup Dutch process cocoa powder, 1 tablespoon Cornstarch, ½ teaspoon Salt
Using a mixer, beat the sugar, eggs, and egg yolk on high speed in a mixing bowl. Beat for 5 minutes, it will become very pale in color. This helps give the brownies a crinkle top.
1 ⅓ cups White granulated sugar, 2 Large eggs, 1 Egg yolk
Add in melted butter, oil, and vanilla. Mix on low speed until just combined. Add in the dry ingredients and mix on low speed until just combined. Use a rubber spatula and scrape the bowl gently.
5 tablespoon Unsalted butter, ⅓ cup Oil, 1 teaspoon Pure vanilla extract
Add in chocolate chips and peanut butter chips. Fold with a rubber spatula.
¾ cup Semi-sweet chocolate chips, ½ cup Peanut butter chips
Pour the brownie batter into the prepared pan. Heat the peanut butter in the microwave for 30 seconds until fully melted and liquidy.
¼ cup Peanut butter
Use a ½ teaspoon and place peanut butter dollops all over the top of the brownie batter. Use all of the peanut butter. Use a butter knife to gently swirl the peanut butter into the brownie batter.
Bake for 40-50 minutes. The brownies are done when edges are set, but the center is barely set. The middle should jiggle. It will keep cooking once pulled out of oven. Let cool completely on a wire rack before serving.