Instructions

• Preheat oven to 350°F (175°C). Line a 9×9-inch baking pan with aluminum foil and spray the foil with cooking spray; set pan aside.
• In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl with an electric mixer), add the 21-ounce cake mix, 1 large egg, 1/2 cup very well softened unsalted butter, and 1 teaspoon vanilla extract. Mix on medium speed until combined, stopping to scrape down the sides as needed.
• The dough should be very thick and come together as a single, softball-like mass that sticks to the paddle. If it does not come together, add up to 3 tablespoons water, 1 tablespoon at a time, mixing after each addition, until the dough forms that mass. Do not add more than 3 tablespoons.
• Transfer approximately half of the dough into the prepared pan. Press it evenly into the bottom of the pan with a spatula or your fingertips to form a flat crust.
• Evenly sprinkle 1 cup semi-sweet chocolate chips over the crust.
• In a medium bowl, whisk together half of the 14-ounce can of sweetened condensed milk and 1/2 heaping cup creamy peanut butter until smooth and fully combined.
• Spoon or pour the peanut butter–sweetened condensed milk mixture over the chocolate chips, keeping a 1/2-inch bare margin from the edges. Smooth lightly with a spatula as needed.
• With the remaining dough, form small ~1-inch pieces or balls and drop them evenly over the top of the filling. Press gently to compact and seal; full coverage is not necessary.
• Bake on the middle rack for 25–27 minutes, or until the top is just set and not jiggly. (A toothpick test will appear wet because the interior is intentionally gooey.)
• Immediately after removing the pan from the oven, use the blunt end of a wooden spoon to poke about 25 holes roughly 1/2-inch deep across the surface.
• Evenly drizzle the remaining sweetened condensed milk from the can over the bars, aiming to fill the holes as best you can.
• Allow the bars to cool uncovered at room temperature for at least 3 hours before serving (overnight is best so the sweetened condensed milk soaks in more). Store airtight at room temperature for up to 1 week.