2 cups sifted confectioners’ sugar
1 1/2 cups smooth peanut butter, preferably all natural
3 tablespoons unsalted butter
1 pound bittersweet or semisweet chocolate
36 1 3/8-inch paper candy cups
1. Combine sugar, peanut butter, and butter in an electric mixer fitted with the paddle attachment. Beat on medium-low speed until combined. Transfer mixture to a pastry bag fitted with a 1/2-inch plain tip, and set aside.
2. Melt chocolate in a double boiler over simmering water. Keep melted chocolate over hot water near work area. Use a small spoon or small paintbrush to coat insides of the paper candy cups with the melted chocolate, making sure to cover the bottom and sides well. Transfer cups to a rimmed baking sheet or muffin tins (which will keep the cups from sliding around). Transfer to freezer until set, about 10 minutes.
3. Remove cold chocolate cups from the freezer. Pipe peanut- butter filling into each cup until three-quarters full. Spoon melted chocolate into each cup to cover. Return to the freezer until set, 15 to 25 minutes. Peanut-butter cups may be served right away, or kept tightly sealed in the freezer for 2 to 3 days. To serve, remove from freezer, peel off paper, and eat cold.