• Preheat the oven to 350 °F. Butter and flour a 9" square cake pan. Cut an 8"x16" piece of parchment paper and line the pan in one direction, leaving a short overhang on each side.

Crumb Topping

• Combine 6 ¾ oz cake flour, 5 ¼ oz brown sugar, ¼ teaspoon salt and 2 teaspoons cinnamon into the bowl of a stand mixer or into a large mixing bowl. Mix on low speed until completely combined. With the mixer running, add 4 oz unsalted butter in chunks and continue to mix until the topping looks like wet sand. Remove from the mixer and transfer to a small bowl by squeezing handfuls to create large clumps. Set aside while you mix the batter.

Batter

• In a small bowl, whisk 8 oz sourdough discard, 1 large egg, 1 large yolk and 1 tablespoon vanilla extract in a small bowl and set aside.
• Sift 6 ¾ oz cake flour, 6 oz granulated sugar, 1 ½ teaspoons baking powder and ¼ teaspoon salt into the bowl of a stand mixer or into a large mixing bowl. With the mixer running on low speed, add 4 oz unsalted butter and 4 oz buttermilk. Mix until completely combined. Scrape the bowl and beater. Mix on medium speed for 2-3 minutes to lighten the batter.
• Scrape the bowl and add the sourdough discard mixture in two batches, scraping between additions. Mix just until combined. At this point you can fold in 2 cups of blueberries or diced apples if you'd like. Spread the batter into the prepared pan.
• Sprinkle on the crumb topping, breaking it into marble sized clumps as you scatter it evenly over the cake batter.
• Bake until a toothpick inserted in the center comes out clean, about 45-50 minutes. Cool 10 minutes in the pan (30 minutes if you put fruit into the cake). Use the parchment to carefully lift the cake out of the pan.
• Cool completely on a wire rack