Make the Topping: Whisk the sugars, cinnamon, salt, and butter in a medium bowl to combine. Add flour and stir with a rubber spatula or wooden spoon until mixture resembles thick, cohesive dough; set aside to cool to room temperature, 10 to 15 minutes.
Make the Cake: Adjust oven rack to upper-middle position and heat oven to 325 degrees F. Spray an 8-inch square baking dish with nonstick cooking spray and line with parchment paper or aluminum foil; allow excess to overhang edges of the dish.
Using an electric mixer on low speed, mix the flour, sugar, baking soda, and salt to combine. With mixer continuing to run 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 a rubber spatula, spread batter into even layer. Break apart the crumb topping into large pea-sized pieces and spread in an even layer over batter, beginning with edges and then working toward the center. (Be sure not to push the crumbs into the batter, just sprinkle on top.) Bake until the crumbs are golden and a wooden skewer inserted into the center of the 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 powdered sugar just before serving. Leftovers can be stored in an airtight container at room temperature for up to 4 days.