Preheat oven to 375 degrees. Spray a 9-inch tart pan with a removable bottom (or a 9-inch cake pan) with cooking spray. Set pan aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugars together until light and fluffy. Add the egg and beat to combine. Add peanut butter and vanilla, beating until fully incorporated.

Combine the flour, cornstarch, baking soda, and salt in a small mixing bowl. With mixer on low, beat in to butter mixture until just combined. Fold in chocolate chips and jimmies until evenly distributed.

Press dough into the prepared pan and bake for 12 to 15 minutes, until lightly golden. A toothpick inserted into the center of the cookie cake should come out clean. Cool completely in pan on a wire rack. Once completely cooled, carefully turn cookie cake out onto a serving plate.

Beat butter on medium speed until light and fluffy, about 2 minutes. Add vanilla and salt and beat to combine. Beat in cocoa powder, then turn mixer to low and gradually add powdered sugar. Beat on high speed for 4 minutes. Add milk 1 teaspoon at a time to achieve a pipe-able consistency.

Pipe frosting onto cooled cookie cake and decorated with sprinkles as desired. Serve and enjoy.