The bars are moist, drippy, gooey, and luscious. They’re rich, fudgy, decadent, and almost brownie-like. A soft chocolate base is topped with chocolate chips and flooded with a mixture of sweetened condensed milk and peanut butter. The sweetened condensed milk takes on creamy, caramely notes as it bakes and pairs perfectly with peanut butter. More chocolate dough tops the sweetened condensed milk and peanut butter mixture. After baking, poke holes in the bars and drizzle sweetened condensed milk into the holes and be patient for the bars to soak it all up.
Preheat oven to 350F. Line a 9×9-inch baking pan with aluminum foil, spray with cooking spray; set aside. I strongly recommend lining the pan for easier cleanup.
To the bowl of a stand mixer fitted with the paddle attachment (or large bowl and electric mixer) add the cake mix, egg, butter, vanilla, and mix on medium speed until combined. Stop and scrape down the sides of the bowl as necessary. Dough is very thick (thicker than cookie dough) and you’ll know it’s the right consistency when it forms a softball-like mass inside the mixer and the ‘softball’ sticks to the paddle. I added 3 tablespoons of water in order to get the dough to combine but all brands of mixes are slightly different in exact size, ingredients, etc. so add water only as necessary.
Turn half the dough out into prepared pan, pressing lightly with a spatula or your fingertips to create and even, flat crust.
Evenly sprinkle with chocolate chips; set pan aside.
To a medium bowl add half the can of sweetened condensed milk, peanut butter, and whisk until smooth and combined.
Turn mixture over the crust and chocolate chips, keeping a 1/2-inch bare margin from the edges as it’s prone to sticking and burning if it touches the edges. Smooth it lightly with a spatula as necessary.
Using the remaining cake dough, form small 1-inch balls or small pieces and evenly drop them over the top. Press gently to seal and compact them noting there will not be full coverage over the sweetened condensed milk layer.
Bake for about 25 to 27 minutes, or until top is just set and not jiggly. A toothpick test doesn’t work because the interior stays gooey and moist, and the toothpick comes out looking wet, even when bars are done.
As soon as you take bars out of the oven, using the blunt end of a wooden spoon, poke the cake in about 25 spots, going down about 1/2-inch.
Evenly drizzle the remaining sweetened condensed milk over the bars, filling up the holes as best you can. It looks like a lot but it does soak in over time. Allow bars to cool, uncovered, for at least 3 hours before serving (overnight is best to allow sweetened condensed milk to soak in even more). Bars will keep airtight at room temp for up to 1 week.