Heat oven to 375 F., and prepare a muffin pan with butter, non-stick spray, or cupcake liners.
In a small saucepan over medium heat, melt the butter.
As the butter cooks, it will crackle and pop. Continue cooking until the crackling stops and the solids have turned a dark golden-brown (amber) color.
Remove pan from heat and pour the butter into a bowl to cool for at least 10 minutes.
Meanwhile, whisk together milk, egg, yolk, and vanilla extract, until fully combined. Whisk in the brown butter until combined.
In a separate, large mixing bowl, combine flour, sugar, baking powder, and salt. Stir to combine, then pour in the wet ingredients and stir gently, until barely combined.
Gently fold in the blueberries, and divide batter evenly into muffin tins.
For Streusel Topping
Use clean fingers to combine butter, flour, and sugar until crumbly. Divide among muffin cups, topping each muffin liberally.
Place pan in oven and bake for 18-20 minutes. Muffins should be golden and crisp on the top, and a cake-tester will come out clean, or with a few moist crumbs.
Let the muffins cool 15 minutes in the pan before removing.