Preheat oven to 350 degrees Fahrenheit. Grease and line two 9 inch cake pans.
In a large bowl, beat yellow cake mix, eggs, oil, milk, peanut butter together until smooth. Divide batter evenly among prepared cake pans and bake for 30 minutes, until a toothpick inserted in the center comes out clean. Transfer to a wire rack to cool completely.
In another large bowl, beat softened cream cheese, softened butter, cocoa powder, vanilla extract, and Brainiac Chocolate Peanut Butter together until smooth. Slowly add powdered sugar and whip until smooth and fluffy, about 3 minutes.
Once the cake has cooled, take a large serrated knife and even the top of each cake round. Save the cake crumbs for later.
To assemble, place a cake round in the center of a cake stand. Frost with an even layer of chocolate frosting. Add a layer of crushed Utz Peanut Butter Pretzel Bites and drizzle chocolate sauce on top.
Add the remaining cake round on top and frost with the remaining chocolate frosting. Then add an even layer of cake crumbs and whole Utz Peanut Butter Pretzel Bites, then drizzle with chocolate sauce.