Blueberry Crumb Muffins

Melt butter over moderately low heat (or in microwave). Whisk in milk, egg, yolk, and vanilla until well combined.
In separate bowl, whisk flour, sugar, baking powder and salt. Add milk mixture and stir until just combined. Gently but thoroughly fold in blueberries.
Divide batter among muffin cups.
Combine all topping ingredients into a bowl and rub together with fingertips until crumbly. Sprinkle evenly over batter in cups.
Bake 375 degrees until golden and wooden pick comes out clean, about 15-20 minutes. Cool in pan on rack for 15 minutes than run knife around edges of muffin cups and carefully remove from pan.