Put roughly chopped white chocolate in a medium heat-proof bowl.
In a small saucepan, combine sweetened condensed milk and 2% milk over medium-low heat until it's just coming to a simmer.
Pour milk mixture over white chocolate and let sit for a minute. Then, stir until chocolate is completely melted and smooth.
Pour sauce into a heat-proof jar or bowl and let cool. Keep in the fridge until ready to use.