4 Tablespoons cold, unsalted butter, cut into 1/2-inch cubes
Preheat oven to 375 degrees F. Line a 12-muffin pan with paper liners.
Melt butter in a small saucepan over medium heat. Stirring continuously, melt and cook butter until little brown bits appear. Butter will quickly begin to brown- once it does, remove from heat
In a medium bowl, 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 large bowl. Add the milk and butter mixture and stir gently to combine. Gently but thoroughly fold in the blueberries. The batter will be really thick.
Divide the batter among muffin cups - I used a 1/4 cup measuring cup. Batter will not really be pourable, so I had to scoop it out from the measuring cup each time.
To make the topping: Combine all of the ingredients in a bowl and rub together with your fingertips until crumbly (this took several minutes). Sprinkle evenly over each muffin.
Bake until golden and a wooden toothpick 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. Store in airtight container (if there are any left ;)).
* Recipe adapted from the fabulous blog Joy the Baker.