Divide your 3/4 cup milk powder into two portions -- one 1/2 cup portion, and one 1/4 cup portion.
Preheat the oven to 250 (F).
Combine the first 1/2 cup portion of milk powder with 1/4 cup flour, 2 tablespoons cornstarch, 2 tablespoons sugar, and 1/2 teaspoon salt in a medium bowl. Whisk together until combined.
Add 4 tablespoons melted butter and toss, using a rubber spatula, until the mixture starts to come together and form small clusters. Spread the clusters on a parchment- or Silpat-lined sheet pan and bake for 20 minutes. The crumbs should be sandy at that point, and your kitchen should smell like buttery heaven. Cool the crumbs completely.
Crumble any milk crumb clusters that are larger than 1/2 inch in diameter, and put the crumbs in a medium bowl. Add the second 1/4 cup portion of milk crumbs and whisk together until it is evenly distributed throughout the mixture.
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Pour the 3 oz. melted white chocolate over the crumbs and toss until your clusters are enrobed. Continue tossing every 5 minutes until the white chocolate hardens and the clusters are no longer sticky. The crumbs will keep in an airtight container in the fridge or freezer for up to 1 month.