• Preheat oven to 325°F. Line an 8x8-inch baking dish with aluminum foil, including up the sides, leaving extra hanging over the edges. Spray with nonstick cooking spray. Set aside.

Topping

• To a medium bowl, add butter, sugars, cinnamon, and salt. Stir to combine. Add the flour and stir until the mixture is completely combined. Be careful not to over mix or it can become like a paste.

Cake

• In the bowl of a stand mixer fitted with 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, yolk, and vanilla; beat on medium speed until light and fluffy, about 1 minute, scraping the bowl as needed.
• Pour 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 batter.
• Bake for 38-40 minutes or until crumbs are golden and a toothpick inserted into the center of the cake comes out clean.
• 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.