Place oven rack in the middle position and preheat oven to 350 degrees F (177 degrees C).
In a large bowl, combine yellow cake mix, melted butter, peanut butter, and eggs; mix until combined.
Reserving 1 ½ cups of the mixture to crumble on top, press the cake batter into a 9X13 baking pan.
In a small pot over medium to medium-low heat, melt chocolate chips, sweetened condensed milk, and butter together. Remove from heat and stir in the vanilla extract.
Spread chocolate mixture over the batter already pressed into the pan, and then crumble the 1½ cups of reserved cake mixture evenly on top.
Bake for 25-30 minutes. Cool completely before cutting into bars. Store in an airtight container.
Notes
Make sure not to turn your pot with your chocolate up too high, or you'll burn the chocolate mixture.
You can add half peanut butter chips and half semi-sweet for a fun twist.
Other additions that work well are nuts, toasted coconut, or butterscotch chips.
Nutritional information is approximate and was calculated using a recipe nutrition label generator.★ Did you make this recipe? Don't forget to give it a star rating below!