For the cookie cake: In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, peanut butter, brown sugar, and granulated sugar, about 1 minute. Add the egg, vanilla, milk, baking soda, and salt, and mix until combined. Slowly add the flour, and mix until the dough just comes together, scraping the sides of the bowl as necessary. Stir in the M&Ms or chocolate chips.
Spray a 9- or 10-inch round cake pan with cooking spray, or line with foil and spray with cooking spray for easy removal and cleanup. Press the dough into the pan. Refrigerate for 30 minutes.
Preheat the oven to 350 degrees F. When the dough is cold, bake for 20 to 25 minutes, until the top is golden brown. Let cool completely before frosting.
For the frosting: Using an electric hand mixer or a stand mixer fitted with the paddle attachment, beat the butter until smooth. Slowly mix in the powdered sugar and cocoa powder. Mix in the salt, vanilla, and 2 teaspoons of the milk. Add the remaining 2 or 3 teaspoons milk as needed to reach the desired consistency.
Place the frosting in a pastry bag fitted with a 1M tip, and frost the outer ring of the cake. Dust with sprinkles if using.
Try substituting the M&Ms with any sort of candy or chocolate chip you love. Peanut butter cups make a fabulous cookie cake!
"Dessert Mashups: Tasty Two-in-One Treats Including Sconuts, S'morescake, Chocolate Chip Cookie Pie and Many More" by Dorothy Kern © Ulysses Press 2014. Reprinted with permission from Ulysses Press. All rights reserved.