Preheat the oven to 350°F (175°C). Line a 13 × 9-inch baking pan with aluminum foil, allowing the foil to extend over the edges for easy removal. Liberally grease the foil with non-stick cooking spray and set aside.

In a large mixing bowl, combine the devil’s food cake mix, egg, and vegetable oil. Stir with a spoon until a soft, cohesive dough forms.

Spread the dough evenly into the prepared pan, pressing it gently into an even layer. Bake for approximately 12 minutes, or until the base is just set. Remove from the oven, keeping the oven on.

Arrange the milk chocolate bars evenly over the warm crust to cover the surface. Sprinkle the milk chocolate chips, semi-sweet chocolate chips, and dark chocolate chips evenly over the bars.

In a small bowl, whisk together the hot fudge sauce and sweetened condensed milk until smooth. Pour the mixture evenly over the chocolate layers.

Return the pan to the oven and bake for 20–22 minutes, or until the center is just set and the surface appears bubbly.

Remove from the oven and allow the bars to cool completely in the pan, preferably overnight, to ensure proper setting.

Once fully cooled, lift the bars out using the foil overhang. Cut into squares and serve.