Line a 9-inch x 9-inch pan with parchment paper & spray with cooking spray.
Combine cake mix, butter, and egg in a stand mixer bowl. Beat with the paddle attachment until it forms into a thick dough. (You can also use an electric hand mixer.)
Press 2/3 of the dough into the prepared baking pan- it will look like there isn't enough to cover the bottom, just keep working it into a thin even layer. It will cover the bottom.
Sprinkle with 1/2 cup of each type of chocolate chips over the dough.
Pour sweetened condensed milk over the top in a thin, even coating.
Separate remaining dough into balls, and evenly distribute over the top of the chocolate chips.
Press dough balls down to flatten a bit.
Sprinkle with remaining chocolate chips.
Bake for 27- 30 minutes until the top is set.
Remove from oven and cool completely before slicing... if you aren't eating warm, right out of the pan.