Bake cake according to box directions in two 8- or 9-inch pans; cool completely.
Beat peanut butter and butter until smooth. Add powdered sugar, vanilla, and 1/2 cup cream; beat to spreadable frosting.
Whip remaining 1/2 cup cream to soft peaks and fold into peanut butter frosting for lighter texture.
Level cakes, spread a thin layer of frosting between layers, then stack.
Frost top and sides with remaining frosting and press crumble mixture around base.
Drizzle melted chocolate over top and sprinkle chopped peanut butter cups.
Chill 30 minutes so it sets before slicing.