Preheat oven to 350°F. Line an 8-by-8-inch square pan with parchment. Gently spray the parchment with non-stick baking spray.
In a large bowl, add melted butter, brown sugar, egg, and vanilla. Whisk until smooth.
Add flour, malted milk powder, baking powder, and salt to the bowl. Using a rubber spatula, stir until it is thoroughly combined (the batter will be very thick).
Fold in the white chocolate chips and sprinkles.
Evenly spread the mixture into the pan. Bake for 25 minutes, or until a toothpick inserted in the center comes out mostly clean.