Preheat & Prepare the Pan:
Preheat oven to 375°F (190°C).
Line a 12-cup muffin pan with paper liners.
Brown the Butter:
Melt butter in a small saucepan over medium heat.
Stir and cook until golden brown bits form, and the crackling subsides. Remove from heat and let cool slightly.
Mix the Wet Ingredients:
In a medium bowl, whisk together milk, egg, egg yolk, and vanilla.
Stir in the browned butter until well combined.
Combine Dry Ingredients:
In a separate large bowl, whisk together flour, sugar, baking powder, and salt.
Combine Wet & Dry Ingredients:
Add the wet ingredients to the dry mixture and stir gently.
Fold in the blueberries carefully.
Make the Topping:
In a small bowl, rub together butter, flour, and sugar with your fingertips until crumbly.
Fill & Bake:
Divide the batter evenly into the muffin cups.
Sprinkle the topping over each muffin.
Bake for 18-20 minutes, until golden brown and a toothpick inserted in the center comes out clean.
Cool & Serve:
Let muffins cool in the pan for 15 minutes, then transfer to a wire rack.
Serve warm or at room temperature.