• 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, 2 teaspoons cinnamon, and 1 teaspoon ground ginger 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

• Combine 1 large egg, 1 yolk, 6 oz buttermilkalf the buttermilk and 1 teaspoon vanilla in a small bowl and set aside.
• Sift 9 oz cake flour, 6 oz granulated sugar, 1 ½ teaspoons baking powder, 1 teaspoon ground ginger, 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 butter and the remaining 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 egg mixture in two batches, scraping between additions. Remove 1 cup of the batter and spread it into the pan. By hand, fold the rhubarb into the remaining batter. Spread the batter with the rhubarb into the pan.
• Sprinkle the crumb topping over the batter, breaking it into marble sized clumps as you scatter it over the cake batter.
• Bake until a toothpick inserted in the center comes out clean, about 45-50 minutes. Cool at least 30 minutes in the pan. Use the parchment to carefully lift the cake out of the pan.
• Cool completely on a wire rack