To make the batter: Put oven rack in upper third of oven and preheat to 375 degrees F. Generously butter muffin pans.
Melt butter in a small saucepan over moderately low heat, then remove from heat. Whisk in milk, then whisk in whole egg, yolk, and vanilla until combined well.
Whisk together flour, sugar, baking powder, and salt in a bowl, then add milk mixture and stir until just combined. Fold in blueberries gently but thoroughly.
Divide batter among 12 muffin cups, spreading evenly.
To make the topping and bake muffins: Rub together all topping ingredients in a bowl with your fingertips until crumbly, then sprinkle evenly over batter in cups.
Bake until golden and crisp and a wooden pick or skewer inserted diagonally into center of a muffin comes out clean, 18 to 20 minutes.
Cool in pans on a rack 15 minutes, then run a knife around edge of each muffin top and carefully remove from cups. Serve warm or at room temperature.
Muffin top pans are available at cookware shops and cooking.com.
Recipe courtesy Gourmet Magazine