Brown butter: In a small saucepan over medium-high heat, brown the butter, stirring constantly until it turns golden and smells nutty. Transfer to a heat-safe bowl and let cool slightly.
Streusel: In a medium bowl, mix together the streusel ingredients until coarse crumbs form. Set aside.
Coffee cake: In a large bowl, whisk together the browned butter, milk, yogurt, eggs, vanilla, mashed bananas, and brown sugar until well combined. Add the dry ingredients and gently fold until just incorporated.
Line and grease an 8x8 baking pan. Spread half of the batter into the pan, sprinkle with a layer of streusel, add the remaining batter, and finish with the rest of the streusel on top.
Bake at 375°F for 55-60 minutes, or until a toothpick inserted in the center comes out clean. Let cool before adding the glaze. Enjoy!
