Line a square 9x9 pan with parchment paper. Set aside.
In a microwave-safe bowl, combine dark chocolate chips with 3 Tablespoons sweetened condensed milk.
Heat for 25 seconds, stir well, and reheat for another 25 seconds as needed until you get a smooth consistent texture.
Fold the mixture out into the lined pan and smooth to an even layer using an offset spatula.
Place in the freezer as you deal with the white chocolate chips.
In a second microwave-safe bowl, combine white chocolate chips with 3 Tablespoons sweetened condensed milk and repeat the same heating procedure as for the dark chocolate chips.
Layer the white fudge layer over the dark chocolate layer and smooth with an offset spatula.
Press in your mini marshmallows and refrigerate at least 4 hours (or overnight) to set.
When ready, remove pan from fridge and remove the fudge from the pan, then the parchment paper.