Prepare cake according to directions on box. Bake in 2 greased and floured 9 inch round pans. Cool completely.
For frosting: Beat together butter, peanut butter, vanilla and salt. Add powdered sugar and enough cream to reach spreading consistency. Do this while cake is cooling.
To assemble: Place one cake layer on a plate, flat side up. Spread with a layer of frosting. If desired, sprinkle with a few finely chopped Reese’s peanut butter cups. Top with remaining cake layer. Frost top and sides of cake. Decorate with halved Reese’s peanut butter cups. Finely chop any remaining peanut butter cups and sprinkle them in the center.
If you make this cake during warmer months and your house is warmer then you will need to keep it refrigerated as the icing may begin to "droop" on you. You may also find that freezing (or refrigerating) the Reese's cups will make them easier to work with. I used mine from the refrigerator, not frozen.