Using either a hand held mixer or a stand mixer, mix softened butter and peanut butter at medium speed until smooth
Add powdered sugar and salt. Mix at lower speed until the sugar is moistened.
Add the vanilla and heavy cream (or canned milk) and beat on high until light and fluffy, a good 10 minutes.
When I made this it was to top chocolate cupcakes. I just used a boxed chocolate cake mix, put a bit of batter in the bottom of the cupcake paper, put a mini Recces peanut butter cup, then fill up to 2/3 full. After they baked, and cooled, I frosted them with this buttercream and topped each with chopped peanut butter cups. They were a HUGE hit at the bake sale.