For the cake:
- 1/ 2 cup butter
- 1 cup rye flour
- 1 cup all purpose flour
- 1/ 2 teaspoon baking soda
- 1/ 2 teaspoon kosher salt
- 2 medium ripe bananas
- 2/ 3 cup sourdough discard
- 2 large eggs
- 1 teaspoon vanilla extract
For the topping:
- 4 tablespoons cold and cut into cubes unsalted butter
- 2/ 3 cup all purpose flour
- 1 1/ 2 teaspoons cinnamon
- 1/ 2 teaspoon freshly grated nutmeg
- powdered sugar for dusting, optional
Make the cake:
Preheat the oven to 350°F with the rack in the middle. Prepare an 8 inch cake pan by buttering and flouring the inside, tapping out any excess flour.
In a medium skillet over medium heat, melt the butter and allow to simmer until all the water evaporates and the butter is browned. Stir occasionally during this process and use sound as a cue; when the butter becomes quiet, it’s done browning. Remove from heat and set aside to cool slightly.
Combine the rye flour, all purpose flour, brown sugar, baking soda, and salt in a large bowl and set aside.
Mash the bananas in a medium bowl with a fork until smooth. Add the sourdough discard, eggs, and vanilla, and whisk to combine. Stir in the warm, but not hot, browned butter. Pour wet ingredients into dry ingredients and fold until mixture comes together and no dry streaks remain; do not over mix. Allow to sit briefly while you make the topping.
Make the topping:
combine the flour, sugar, cinnamon, and nutmeg in a medium bowl. Add the butter and toss to combine, using your fingertips to press butter cubes into dry mixture until you are able to form crumbs with your hands. Some bits will be sandy, others will be large crumbs.
Pour cake batter into prepared pan, and top with crumb topping. Bake for 35-45 minutes, rotating halfway through, or until a toothpick inserted into the center comes out just clean. Take care not to over-bake; slightly under is better than slightly over.
Cool cake on rack until just warm to the touch, then dust with powdered sugar if using. Slice and serve.