• Preheat the oven to 325°F. Line an 8×8-inch baking dish with aluminum foil, including up the sides, leaving extra hanging over the edges. Spray with nonstick cooking spray. Set aside.

Topping

• In a medium bowl, add butter, sugars, cinnamon, nutmeg, and salt. Stir to combine. Add the flour and stir until the mixture is completely combined. Be careful not to overmix.

Cake

• To the bowl of a stand mixer fitted with a paddle attachment on low speed, add flour, sugar, salt, and baking soda. Mix to combine. Add butter pieces one at a time, mixing until there are no visible butter pieces and it has a very fine crumb, about 1 minute.
• Add buttermilk, egg, egg yolk, vanilla, and mashed bananas. Beat on medium speed until light and fluffy, about 1 minute, scraping the bowl as needed.
• Pour the batter evenly into the prepared pan, spreading with a spatula if needed.
• Using your hands or a fork, break up the topping into pieces. Spread the topping in an even layer on top of the banana batter.
• Bake for 40-45 minutes, or until crumbs are golden, and a toothpick inserted into the center of the cake comes out with a few crumbs but no wet batter.
• Let the cake cool in the pan on a wire rack for 30 minutes. Then, using the excess aluminum foil, lift the cake out of the pan to continue cooling.
• Dust with confectioners’ sugar just before serving.