Heat oven to 350º. Butter two 9-inch round cake pans.  Sprinkle flour on the inside, tap out excess and line bottom with parchment paper.

Whisk together flour, cocoa powder, baking powder, baking soda and salt in a medium bowl and set aside.

Using a stand mixer fitted with the paddle attachment, beat butter on medium speed until smooth and creamy. Add sugar and continue beating for about 2 minutes, until it is thoroughly blended into the butter. Add eggs and yolks one at a time, beating for 1 minute after each addition.  If needed, scrape down the sides and bottom of the bowl. Beat in vanilla. Reduce speed to low and alternately add dry ingredients and buttermilk, with the dry ingredients in 3 portion and the buttermilk in 2, starting and ending with the dry ingredients. Mix only until just blended into the batter.  Scrape down bottom and sides of bowl.

Divide batter evenly between two pans and bake for 26 to 30 minutes, until the cakes feel springy to the touch. Allow cakes to cool in pans on rack for about 5 minutes, then remove from pans and allow to cool to room temperature.  At this point, cake layers can be wrapped airtight and left at room temperature overnight or frozen up to 2 months.

While cake is cooling, make frosting by combining confectioner’s sugar, peanut butter, butter, vanilla and salt in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium-low speed until creamy, scraping down sides and bottom of bowl as needed. Add the cream, and beat on high speed until smooth.

Freeze the Reese’s Cups while filling and frosting the cake, then chop/halve them as desired and use them to top the cake.