Preheat the oven to 325 F and line two baking sheets with parchment paper.
Toss the cornflakes with the melted butter, brown sugar, and salt. Spread the mixture on one baking sheet and bake for 8 minutes, then cool completely.
Increase the oven temperature to 350 F. Beat the softened butter, brown sugar, and granulated sugar until creamy, then mix in the egg and vanilla.
Whisk the flour, milk powder, baking soda, and salt in a separate bowl. Add the dry ingredients to the butter mixture and mix just until combined.
Fold the white chocolate chips and three-quarters of the cooled cereal crunch into the dough. Divide the dough into 12 portions and press the remaining crunch onto the tops.
Arrange the cookies at least 2 inches apart on the prepared baking sheets. Bake for 10 to 12 minutes, until the edges are golden and the centers remain soft.
Cool the cookies on the baking sheets for 10 minutes before transferring them to a cooling rack.