Design Eat Repeat

0 Preheat oven to 350ºF. While preheating, be sure to read tips & tricks in post above for recipe sucess.
26 In a large mixing bowl, beat together unsalted butter [adjustable](½ cup, i.e. 1 stick), light brown sugar (¾ cup), and granulated sugar (¼ cup)[/adjustable] on medium speed for 30-60 seconds, or until combined. Mix in peanut butter [adjustable](¾ cup).[/adjustable]
22 Stir in the whole egg ([adjustable]1 large), egg yolk (1 large), and vanilla extract (1 teaspoon), followed by the baking soda (½ teaspoon) and salt (¼ teaspoon[/adjustable]).
23 Mix in all-purpose flour [adjustable](1 ½ cups) and cornstarch (1 tablespoon), followed by the semi-sweet chocolate chips (½ cup) and peanut butter chips (½ cup)[/adjustable]. Mix the dough until well-combined.
24 Generously grease a cookie cake pan. If you plan to invert the cake onto a tray (rather than serving in the pan), I highly recommend lining the bottom with parchment paper in order for the cookie cake to invert out of the pan easily.
25 Spread dough into an even layer in pan. If desired, press a few extra chocolate chips on the top for added presentation.Bake for 16 to 18 minutes, or until edges are slightly golden brown. Do not overbake; it might look a little under-baked, but the residual heat from the pan will continue to bake the cookie even after you take it out of the oven. Cool completely.*If baking in a 9x13 pan, bake for 14-16 minutes.
27 Use a piping bag and large star tip to create a simple buttercream border. Immediately after piping, top with the extra chocolate and peanut butter chips, or miniature peanut butter cups.
To Make Chocolate Peanut Butter Buttercream Frosting:
30 Beat the softened unsalted butter [adjustable](½ cup, i.e. 1 stick), vanilla extract (2 teaspoons), and salt (¼ teaspoon)[/adjustable] on medium-high speed for 1 minute until smooth. Turn the mixer to low speed then slowly add the unsweetened cocoa powder [adjustable](½ cup) and powdered sugar (2 cups)[/adjustable]. You may need to stop and scrape down the sides. One tablespoon at a time, add milk [adjustable](2 tablespoons), followed by the peanut butter (3 tablespoons)[/adjustable].Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency. This will make just enough frosting to pipe a border along the edges. If you plan to write or decorate the center of the cake, double the recipe. Makes 1 ½ cups frosting.