Adapted from Cook's Illustrated
Serves 8 to 10
For the crumb topping:
For the cake:
Begin by making the crumb topping. Mix sugars, cinnamon, salt, and butter in a bowl to combine. Add flour and stir until mixture forms a thick, slightly crumbly dough. Set aside while you assemble the cake.
Preheat oven to 325 F. Spray an 8 x 8 baking dish with nonstick spray and line with a length of parchment paper slightly smaller than the width of the dish. Allow parchment to overhang sides of dish. Spray parchment with nonstick spray as well.
In the bowl of an electric mixer fitted with the paddle attachment, mix flour, sugar, baking soda and salt on low speed to combine. With mixer on low, add butter one piece at a time, beating until mixture resembles moist crumbs with no visible chunks of butter, 1 to 2 minutes. Add egg, yolk, vanilla, and buttermilk and beat on medium-high speed until light and fluffy.
Transfer batter to baking pan and spread into an even layer. Roll crumb topping into large chunks and spread in an even layer over batter. Bake until a wooden skewer inserted into center of cake comes out clean, 35 to 40 minutes. Cool on wire rack at least 30 minutes before removing cake from pan by lifting parchment overhang.
Cake can be wrapped in plastic wrap and stored at room temperature for up to 2 days.