How to Make It
Heat the oven to 350°F. Line a standard 12-cup cupcake tin with paper liners. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Using an electric mixer and a large bowl, beat the butter and sugar on medium-high until fluffy, 2 to 3 minutes. Beat in the vanilla, then the eggs one at a time, scraping down the sides of the bowl as necessary.
Reduce mixer speed to low. Add the flour mixture in 3 additions and the buttermilk in 2 additions, beginning and ending with the flour mixture. Mix just until combined (do not overmix).
Scoop half the batter into the prepared pan, filling each cup halfway. Add 1 peanut butter cup to the center of each cupcake, then scoop the remaining batter into the cups. Bake until a toothpick inserted in the center, just to the side of the peanut butter cups, comes out clean, about 15 to 20 minutes. Cool the cupcakes in the pan for 15 minutes, then turn out onto a rack to cool completely.
To prepare the frosting, beat the butter and the peanut butter with an electric mixer on medium until smooth. Add the confectioners’ sugar and salt and beat until fluffy. Transfer the frosting to a pastry bag, fitted with a large star tip. Frost the cupcakes in a decorative pattern and sprinkle with additional salt, if you like.