Preheat the oven to 325 degrees F. Line a 13x9 baking dish with parchment paper.
Place the cake mix, peanut butter, melted butter and eggs in the bowl of your stand mixer and beat on low speed for 1 minute. Stop the machine and scrape down the sides of the bowl with a rubber spatula. The mixture will be thick.
Reserve 1 1/2 cups for the topping. Transfer the remaining crust mixture to the pan. Using your fingertips, press the crust evenly over the bottom of the pan so that it reaches all sides. Set aside.
For the filling, place the chocolate chips, condensed milk, and 2 tablespoons butter in a medium pot over low heat. Stir and cook until the chocolate is melted and the mixture is well combined, 3 to 4 minutes. Remove the pan from the heat and stir in the vanilla.
Pour the chocolate mixture over the crust, and spread it evenly with the rubber spatula so that it reaches the sides of the pan. Using your fingertips, crumble the reserved crust and scatter it evenly over the chocolate.
Bake until the top crust is light brown, 25 to 30 minutes. Remove the pan from the oven and cool on a wire rack for 30 minutes.
Cut into 16 pieces and serve.
Store at room temperature for 3 days or in the refrigerator for up to one week.