Preheat oven to 350F (175C) and prepare a 9x13 glass baking dish¹ by spraying with baking spray. Prepare Brownie batter first:
In a large, microwave-safe bowl, combine butter and ½ cup semisweet chocolate chips. Heat in 20-second increments (stirring well in betweeuntil chocolate and butter are melted and mixture is smooth.
Add cocoa powder and sugars and stir well.
Stir in eggs, egg yolk, vanilla extract, and salt.
Add flour and stir well. Stir in mini chocolate chips, if using. Set aside while you prepare peanut butter blondie batter.
Peanut Butter Blondies
Using an electric mixer or stand mixer, cream together butter, peanut butter, and sugars until creamy and well-combined.
Add egg and vanilla extract and stir well.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually add flour mixture to peanut butter mixture and stir until well. Add milk and stir until ingredients are completely combined.
Use a spoon or spatula to stir in Reese’s pieces candy pieces, if using.
Alternate dropping heaping spoonfuls of brownie batter and peanut butter blondie batter into your prepared pan. Use a knife to swirl and make sure the batter is evenly distributed. Top with additional candy pieces, if desired.
Transfer to 350F (175C) and bake for 35-40 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs (no wet batter).
Allow brownies to cool in pan before slicing and serving.
¹You may use a metal pan instead, just keep in mind that the peanut butter brownies may cook faster so start checking them sooner than the 35 minutes recommended in the recipe.