Preheat oven to 325°F. Set aside an ungreased 9"x13" metal pan.
Stir and combine cake mix, melted butter, peanut butter, and eggs in a large bowl using an electric mixer or a wooden spoon. The batter will be thick. Press this into the pan, reserving 1 1/2 cup of the mixture to crumble on top.
In a small pot, melt chocolate chips, sweetened condensed milk, and butter. Remove from heat and stir in the vanilla.
Spread chocolate mixture over the cookie crust in the pan, and then crumble the 1 1/2 cups of reserved cake mixture on top evenly.
Bake for 20-25 minutes. Cool completely before cutting into bars.
These cookie bars are amazing when eaten warm, but are just as good cooled. Serve with a tall glass of milk, and enjoy. You know you want one!
Notes
You could, though I recommend sticking with what the recipe calls for. Doing so delivers the sweetness and texture you'll enjoy. Evaporated or fat-free milk is thinner and not as sweet. Healthier? Yep. But not as tasty.
For a texture change, use crunchy peanut butter. Totally delicious. In fact, that's what I used! For a slightly healthier option, use all-natural peanut butter.