Soak cereal: In a medium mixing bowl, combine 2 cups milk with 2 cups of cereal. Let soak for at least 1 hour, then strain to get 1 cup of cereal milk.
Brown butter: Melt 4 tablespoons of butter until light amber, then set aside to cool.
Preheat oven to 350°F. Whisk together eggs, cereal milk, coconut milk, maple syrup, and vanilla. Add rice flour, sugar, baking powder, salt, and browned butter. Pour batter into prepared pan.
Crumble, bake, and serve: Crush 3 cups of cereal, mix with melted butter, sugar, and salt. Top batter with cereal mixture. Bake for 1 hour, then cool, cut into squares, and serve.