Make the milk crumbs: Heat the oven to 250°F. In a medium bowl, whisk together 1/2 cup of the dry milk with the flour, corn starch, sugar, and salt. With a spatula, stir in the butter until small clumps start to form. Spread the mixture on a parchment lined baking sheet and bake 20 minutes until sandy. Cool. Return the crumbs to a bowl, stir in the other 1/4 cup of dry milk and pour the white chocolate into the crumbs, mixing until evenly coated. Set aside.