Brown-Butter Banana Bread Donuts

Ingredients
CRUMB TOPPING
1¼ cups all-purpose flour
1/2 cup brown sugar, lightly packed
1/2 cup granulated sugar
1½ teaspoons ground cinnamon
1/4 teaspoon salt
1/2 cup (1 stick) unsalted butter, cold and cut into small cubes
DOUGHNUTS
2 cups all-purpose flour
3/4 cup granulated sugar
1 teaspoon baking soda
3/4 teaspoon ground cinnamon
1/2 teaspoon freshly grated nutmeg
1/2 teaspoon salt
1/2 cup brown butter
2 large eggs
1 teaspoon pure vanilla extract
1/4 cup buttermilk
1¼ cups mashed banana (from about 3 medium bananas)
GLAZE
2 cups powdered sugar
1 pinch salt
3 tablespoons unsalted butter, melted
1 teaspoon pure vanilla extract
3 to 4 tablespoons whole milk, divided


Preparation
Place a rack in the center of the oven and preheat to 350°F. Grease two mini doughnut pans with nonstick cooking spray. Line a rimmed baking sheet with parchment paper.

For the crumb topping:

In a medium bowl, whisk together the flour, sugars, cinnamon and salt. Add the butter and, using your fingers, quickly blend the butter into the flour mixture until there are bits the size of oat flakes but most is incorporated into the mixture. Spread into an even layer on the prepared baking sheet. Bake for 18 to 20 minutes, stirring twice during baking, until lightly brown. Let cool completely.

For the doughnuts:

In a large bowl, whisk together the flour, sugar, baking soda, cinnamon, nutmeg and salt.

In a medium bowl, whisk together the browned butter, eggs, vanilla and buttermilk. Whisk in the mashed bananas. Add the mixture all at once to the flour mixture. Fold together, making sure to scrape the bottom of the bowl to get hidden pockets of flour, until just combined; do not overwork. Spoon the batter into the prepared doughnut pan.

Bake for 12 to 15 minutes, until a toothpick inserted in a doughnut comes out clean. Let cool in the pan for 5 minutes before inverting onto a wire rack to cool completely.

For the glaze:

In a medium bowl, whisk together the powdered sugar and salt. Whisk in the butter, vanilla and 2 tablespoons of milk. Add more milk, if needed, for a thin and very spreadable consistency.

To assemble:

Once the doughnuts are completely cool, use a butter knife to spread each doughnut with glaze and immediately add a generous amount of crumble topping to each donut. Let set for 15 minutes before serving. The doughnuts are best the day they're made, but they will keep for up to 3 days in an airtight container at room temperature.