Preheat oven to 375° F (190° C). Place paper liners in a 12-cup muffin pan. Combine the topping ingredients using your fingers until the mixture is the texture of coarse sand. Set aside.
Combine the melted butter, milk, egg, egg yolk and vanilla extract. Set aside. Combine the flour, maple flakes, baking powder and salt. Add this dry mixture to the milk mixture and stir just until moistened.
Gently fold in the blueberries. Spoon the batter into the muffin cups, and sprinkle the crumble topping over each muffin.
Bake 18 to 20 minutes or until a toothpick inserted in the centre comes out clean.
Cool 10 minutes in the pan before transferring to a wire rack.