Melt the butter in a small saucepan on medium heat. Once melted let it cook 5 more minutes until browned with brown flecks in it. Remove and let cool.
In a mixing bowl, whisk milk, egg, egg yolk, and vanilla.
Add the brown butter and stir to combine.
Add the flour, sugars, baking powder and salt.
Gently fold in mashed bananas.
Divide batter among 12 muffin cups lined with cupcake liners.
To make the topping, combine all of the ingredients in a bowl and mix with a pastry cutter or fork until crumbly. Sprinkle evenly over the batter in the muffin cups.
Bake until golden brown 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 pan and serve warm with butter.
Recipe by Mandy's Recipe Box at https://mandysrecipeboxblog.com/browned-butter-banana-muffins/