My son looooves these! I use smooth peanut butter in the mixture and the buttercream and top them with miniature Reese's Pieces®! You can also use a basic chocolate cupcake recipe and add the peanut butter, too. Can't fail to impress!
I have made them with and without cocoa powder and my son prefers them to have a chocolate feel but they taste nice both ways. You can use chunky peanut butter but they have a much heavier feel to them and are more grainy.