Put a rack in the upper third of the oven and preheat oven to 375 degrees F. Line muffin pan with paper or foil liners.
Melt butter in a small saucepan over medium heat. Keep an eye on the butter. Melt and cook down the butter until little brown bits appear in the pan. The crackling will subside and butter will begin to brown fairly quickly after that. Keep a close eye. Remove from heat and cool slightly.
Whisk milk, egg, yolk and vanilla until combined. Add the brown butter and stir to combine.
Whisk together flour, sugar, baking powder and salt in a medium bowl. Add milk and butter mixture all at one and stir gently to combine. Gently but thoroughly fold in the blueberries.
Divide the batter among muffin cups and spread evenly.
To make the topping combine all of the ingredients in a bowl and rub together with your fingertips until crumbly. Sprinkle evenly over the batter in the cups.
Bake until golden and crisp and a wooden pick inserted into the center of a muffin comes out clean, about 18-20 minutes. Cool in pan on a rack for 15 minutes then remove from the pan. Serve warm or at room temperature.