Make the Topping: Whisk the 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.
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 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 rubber spatula, spread batter into even layer. Break apart the crumb topping into large pea-sized pieces and spread in even layer over batter, beginning with edges and then working toward 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 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 powdered sugar just before serving. Leftovers can be stored in an airtight container at room temperature for up to 4 days.
Flour: Do not substitute all-purpose flour for the cake flour, as doing so will create a dry, tough cake.
Buttermilk: You can substitute the same amount of plain yogurt for the buttermilk. Using powdered buttermilk is not recommended; it will create a thinner batter and the crumbs may sink to the bottom.
Pan: A square metal baking pan is recommended; if you use a glass pan, the cake may take a little longer to finish baking.
Doubling: This recipe can be doubled and baked in a 9x13-inch pan. Increase the baking time to 40 to 45 minutes.
Flavors/Mix-ins: Fresh blueberries (gently fold 1 cup into the batter) or mini chocolate chips (fold ½ cup into the batter) would work great!
Drizzles/glazes: A drizzle of salted caramel sauce or melted white chocolate over the top would be a nice addition, or a cream cheese glaze, as well!
Storage: The crumb cake can be stored in an airtight container at room temperature for up to 4 days.
Freezing Instructions: Once completely cool, wrap the crumb cake in plastic wrap, then in foil, and place in a ziplock freezer bag for up to 3 months. Thaw at room temperature before serving.