Preheat oven to 350ºF and grease 9" springform pan, (or use square pan or pie plate – cast iron skillet may work); set aside.

In medium bowl whisk together flour, baking soda and salt; set aside.

In large bowl using electric mixer or stand mixer fitted with paddle attachment, cream together butter and cashew butter on medium-high speed for 2-3 minutes. Add in sugar, mixing on high speed for about 20 seconds. Add in egg, egg yolk and vanilla mixing on medium speed until combined.

Slowly incorporate about half of the bowl of flour, add in milk with mixer running then add in the rest of the flour. Fold in the chocolate chips until thoroughly combined.

Pour batter into prepared pan and use slightly damp hands to spread top of batter smooth; top with more chips if desired.

Bake cookie cake for 12 minutes then cover pan with foil and bake for another 10-12 minutes. Edges will turn slightly golden. Insert toothpick into center of cake to check for doneness; toothpick should come out almost clean – no wet batter. Be careful not to over-bake or cookie cake will be dry.

Allow cookie cake to cool completely – about 1 hour – before frosting and slicing.

To make frosting: in medium bowl using electric mixer or stand mixer fitted with paddle attachment, add peanut butter and softened butter; beat on medium-high speed until combined. With mixer running on low speed add in cocoa powder and powdered sugar while slowing adding milk 1/2 tablespoon at a time, then mixing on medium-high speed until frosting is light and fluffy. Transfer frosting to piping bag using Wilton 1M star tip and pipe frosting along edge of cookie cake. Top with sprinkles if desired before slicing and serving.

Cookie cake can be made a day in advance and wrapped tightly. Add frosting the day of serving. Enjoy!