Pre-heat oven to 350 degrees

Line a 9x9 pan with parchment paper & spray with cooking spray

Combine cake mix, butter & egg in mixer bowl & beat with paddle attachment until mix forms into a thick dough

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)

Sprinkle with 1/2 cup of each type of chocolate chips

Pour sweetened condensed milk over the top in a thin, even coating

Separate remaining dough into balls & evenly distribute over the top of the chocolate chips

Press down to flatten a bit

Add remaining chocolate chips

Bake 27- 30 minutes

Cool completely before slicing if you aren't eating warm, right out of the pan.