1 box chocolate cake mix
16 regular sized peanut butter cups, unwrapped and frozen
1/2 cup butter, softened
1/2 cup sweetened condensed milk
1/2 cup peanut butter M&Ms
1 large egg
1 teaspoon vanilla extract
Preheat oven to 350°F and lightly grease an 8×8-inch baking dish.
In a large bowl, mix together cake mix, egg, butter, and vanilla until well combined. Press half the dough into prepared baking dish.
Arrange frozen peanut butter cups over the layer of dough, and press the remaining dough evenly over the top. Drizzle condensed milk over the top and sprinkle with peanut butter M&Ms (or Reese’s Pieces).
Bake for 30 minutes. Bars will appear under-done; they will firm up as they cool. Let cool completely before cutting into squares.