Don’t be tempted to substitute all-purpose flour for the cake flour, as doing so will make a dry, tough cake. When topping the cake, take care to not push the crumbs into the batter. This recipe can be easily doubled and baked in a 13 by 9-inch baking dish. If doubling, increase the baking time to about 45 minutes. Cooled leftovers can be wrapped in plastic wrap and stored at room temperature for up to 2 days.
To make the topping: Whisk sugars, cinnamon, salt, and butter in medium bowl to combine. Add flour and stir with rubber spatula or wooden spoon until mixture resembles thick, cohesive dough; set aside to cool to room temperature, 10 to 15 minutes.
To make the cake: Adjust oven rack to upper-middle position and heat oven to 325 degrees. Cut 16-inch length parchment paper or aluminum foil and fold lengthwise to 7-inch width. Spray 8-inch square baking dish with nonstick cooking spray and fit parchment into dish, pushing it into corners and up sides; allow excess to overhang edges of dish.
In bowl of standing mixer fitted with paddle attachment, mix flour, sugar, baking soda, and salt on low speed to combine. With mixer running at low speed, add butter one piece at a time; continue beating until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes. Add egg, yolk, vanilla, and buttermilk; beat on medium-high speed until light and fluffy, about 1 minute, scraping once if necessary.
Transfer batter to baking pan; using rubber spatula, spread batter into even layer. Break apart crumb topping into large pea-sized pieces between your thumb, pointer, and middle fingers and spread in even layer over batter, beginning with edges and then working toward center. Bake until crumbs are golden and wooden skewer inserted into center of cake comes out clean, 35 to 40 minutes. Cool on wire rack at least 30 minutes. Remove cake from pan by lifting parchment overhang. Dust with confectioners’ sugar just before serving.