Line a 9 x 9 inch pan with foil and grease with butter or cooking spray.
Using a microwave safe mixing bowl, combine white chocolate chips with 6-7 Tablespoons of the sweetened condensed milk and set aside.
In a different microwave safe mixing bowl, combine semi-sweet chocolate chips with the remaining condensed milk and microwave for 30 seconds.
Take out, stir, and microwave again for 20-30 seconds.
Stir well and then pour into the bottom of the 9 x 9 pan.
Next, melt the white chocolate chips/condensed milk mixture in the microwave for 30 seconds.
Remove, stir, and put back in microwave for an additional 15-20 seconds.
Remove from microwave and spread over semi-sweet chocolate mixture in the pan. (I would wait a few minutes before adding the white chocolate layer so that the semi-sweet mixture has time to harden just slightly.)
Cover the top with the bag of marshmallows and pat down on the top.
Stick in the refrigerator so that it will harden….takes 6-8 hours.