1 cup powder sugar
1 cup creamy peanut butter
4 tablespoons unsalted butter, at room temperature
1/2 teaspoon vanilla extract
1/4 teaspoon salt
1/3 cup heavy (whipping) cream
Blend the first 5 ingredients together, until well combined. Add the heavy cream until you get the consistency that you like.
I did find that the filling thickened up a little after sitting in the cupcakes.
Original recipe: http://cook-eat-love.com/2010/03/reeses-chocolate-peanut-butter-cake/