Preheat oven to 350 degrees F. and spray two 9 inch cake pans with cooking spray and the bottoms lined with parchment paper.
Heat the milk in a microwave safe bowl until hot, about 45-60 seconds. Remove and add chocolate chips. Let sit for 1 minute then stir to melt.
In a stand mixer, mix the flour, cocoa powder, sugar, salt and baking soda to combine. Slowly add the eggs, sour cream, oil and melted chocolate mixture until combined. Divide batter between the two prepared cake pans. Bake for about 30 minutes, or until baked through. Remove and let cool. Remove cakes from pans.
To prepare cookie dough, beat butter and peanut butter until creamy and smooth. Add sugar, brown sugar, vanilla, flour and milk mixing until thick and cookie dough consistency.
To prepare frosting, beat butter until creamy. Slowly add powdered sugar and cocoa until thick. Beat in melted chocolate. Stack one cake on top of a cake plate and top with peanut butter cookie dough layer. Top with second cake and Frost entire cake. Top with halved Reeses Peanut Butter Cups.