Sift together flour, sugar, baking powder, salt and cocoa powder.
Mix melted butter, egg and vanilla.
Pour buttermilk, peanut butter cups and butter mixture into dry ingredients.
Mix until just combined. Do not overmix.
Put batter into greased mini doughnut pan. I piped it into the pan using a pastry bag.
Bake at 350 degrees for 6-8 minutes. Do not overbake.