In a large bowl with an electric mixer, beat butter and peanut butter until light and creamy. Add in vanilla and mix well. Add in half of the powdered sugar, mixing well to combine, followed by half the cream, mixing well to combine. Repeat with remaining powdered sugar and cream. If needed, add more cream, a tsp at a time, for a thinner frosting consistency. Assemble and frost cake. If desired, press Reese's Pieces along the base of the cake. Once frosted, place in fridge to chill.