You will use the first six ingredients for the crumb topping. Whisk the sugars, cinnamon, salt and using a fork mix in the butter until the mixture is crumbled. Place the bowl in the refrigerator while you mix the cake ingredients.
Adjust oven rack to upper-middle position and preheat oven to 325 degrees.
Cut a 16" length piece of parchment paper and fold lengthwise to 7" width.
Spray a 8" square baking dish with nonstick spray and fit parchment into dish, pushing it into corners up sides; allow excess to overhang edges of dish.
In a bowl of standing mixer mix flour, sugar, baking soda and salt on low speed to combine.
With a mixer running a low speed, add butter pieces one piece at a time; continuing beating until mixture resembles moist crumbs, with no visible butter chunks, remaining - about 1-2 minutes. Add egg, 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 an even layer.
Remove crumble topping from the refrigerator. With a fork break up the crumbs with a fork then with your fingers, sprinkle crumb topping evenly over the batter beginning with the edges and then working toward the center.
Very gently press the crumb topping on the batter.
Bake until the crumble topping is golden brown and a cake tester inserted into the center of the cake comes out clean - about 35-40 minutes.
Cool on a wire rack for at least 30 minutes then sift powdered sugar over the top. Gently remove cake from the pan by lifting parchment overhang.