Line a baking sheet with parchment paper.

In a medium saucepan over low heat, melt the butter with the peanut butter. Stir in the vanilla. Remove from the heat. Stir in the sugar until smooth. (The mixture will be the texture of thick dough.) While still warm, use your hands to shape the dough into balls by the tablespoonful. Place on the baking sheet.

In a saucepan over medium-low heat, melt the chocolate, stirring constantly. Remove from the heat and use a toothpick to dip balls halfway in chocolate. Place back on the baking sheet. Refrigerate until set, about 1 hour.

Prepare brownie-mix batter according to package instructions. Pour half of the brownie batter into a 9-inch by 9-inch baking dish.

Top with peanutbutter cups. Spread the remaining half of the brownie batter evenly over peanut-butter cups. Bake according to package instructions. Let cool completely, and then cut into squares.