In a microwave safe bowl; combine white chocolate chips with 5 Tablespoons sweetened condensed milk.
Line a square 9x9 pan (or 8x8) with foil and generously grease with butter or shortening.
Microwave white chocolate 30 seconds. Stir until smooth. Spread mixture on top of dark chocolate layer. (This will take some finesse to spread into a thin layer)
Refrigerate at least 6 hours to set.