Preheat oven to 350°F. Line a 9x13-inch pan with foil and spray well with nonstick cooking spray.
Place cake mix, egg, and butter in the bowl of a stand mixer fitted with the paddle attachment. You can also use a hand mixer but it will take longer (just keep mixing!)
Press 2/3 of the batter in the bottom of the prepared pan. It will be a thin layer.
Top with the can of sweetened condensed milk, leaving a 1-inch border around the edges. If it's touching the foil it has more chances of bubbling up and/or sticking.
Sprinkle chocolate chips over sweetened condensed milk.
Press tablespoon sized balls of the remaining dough between your hands and place on top the chocolate chips. It won't be a solid cover. Try to make the edges as solid as possible.
Bake for 20-30 minutes, or until golden brown and milk is bubbling up. Cool completely, then chill until set so you can remove the foil and slice. These won't slice nicely until they are completely chilled.
Store in a single layer in an airtight container for up to 3 days.