I love this recipe because it's a great way to use up languishing bananas and they're great for a party because you can make them ahead. They taste like banana bread but are shaped like doughnuts, which makes everything better.
Technique tip: If you don't have a special doughnut pan, you can use the doughnut batter in a waffle iron and just make waffles! You can still use the icing and crumb topping, too!
Swap option: Swap half of the all-purpose flour for whole-wheat flour to make these more whole grain. You could also sub yogurt for the buttermilk.
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.
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.