2 1/2 cups confectioners' sugar
1 1/2 sticks plus 2 tablespoons unsalted butter, softened
1/3 cup heavy cream
1 1/2 cups creamy peanut butter
1 teaspoon pure vanilla extract
8 ounces bittersweet chocolate, chopped
1/2 cup cocoa nibs
1. Place sugar in a large bowl; set aside. Combine butter, cream, peanut butter, and vanilla in a saucepan over medium heat. Cook, stirring, until butter and peanut butter are melted and mixture just starts to boil. Pour over sugar in bowl and whisk until smooth. Transfer to an 8-inch-square baking dish and smooth surface. Place parchment or plastic wrap directly on surface of mixture and refrigerate until set, about 2 hours.
2. Roll heaping teaspoonfuls of peanut-butter mixture into 1-inch balls. Melt chocolate, stirring until smooth, in a bowl set over a saucepan of simmering water; remove from heat. Working with 1 ball at a time, dip into chocolate and remove with a fork, letting excess chocolate drip off; transfer to parchment-lined baking sheets and immediately sprinkle top and sides with 1/4 to 1/2 teaspoon cocoa nibs. Refrigerate until set, about 30 minutes. Truffles can be made 1 week ahead and stored in a single layer, covered, in refrigerator.