• Preheat oven to 325°F and adjust an oven rack to the upper middle position. Line an 8x8-inch square pan with parchment paper. Set aside.

Crumb Topping

• In a medium bowl, whisk ⅓ cup granulated sugar, ⅓ cup brown sugar, ¾ teaspoon cinnamon, ⅛ teaspoon salt, and 1¾ cup flour.
• Pour ½ cup melted butter over the mixture and mix thoroughly. Mixture will be thick and doughy. Set aside.

Cake

• With a hand or stand mixer, combine the 1¼ cup flour, ½ cup sugar, ¼ teaspoon baking soda, and ¼ teaspoon salt.
• With the mixer on low, add 6 tablespoons butter one piece at a time. Mix until flour mixture resembles moist crumbs with no visible butter chunks remaining, about 1-2 minutes.
• Add 1 egg, 1 yolk, 1 teaspoon vanilla, and ⅓ cup buttermilk. Beat at medium for 1-2 minutes until the batter becomes light and fluffy, scrape down the sides of the bowl if necessary.
• Spread the cake batter evenly into the prepared pan. Top the batter with an even layer of pea-sized crumble topping pieces.
• Bake the cake for 35-40 minutes or until golden brown and a toothpick inserted comes out clean and crumb-free. Cool on a wire rack for at least 30 minutes.
• Remove the cake from the pan by lifting it out of the pan by the edges of the parchment paper and dust it with powdered sugar.