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1
Make the crumb topping first. Combine the sugars, cinnamon, and salt in a mixing bowl. Add the melted butter and stir to combine. Stir in the cake flour, until the dough is thick and cohesive (I usually use my hands for this step). Set aside and allow to cool to room temperature. The crumb will be ready by the time you are done mixing the cake batter.
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2
Make the cake batter next. Start by preheating the oven to 325 F, Spray a 9x9 baking pan with baking spray (or grease and flour the pan) and set aside.
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3
Combine the flour, sugar, baking soda, and salt in a large bowl. Cut the butter into the flour mixture with a pastry cutter, until it resembles coarse peas with no visible chunks of butter remaining. Add the egg, egg white, vanilla, and the buttermilk. Beat with a hand mixer on medium-high speed until the batter is light and fluffy. Pour the batter into the cake pan and smooth into a nice, even layer.
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4
Go back to the crumb topping. Use a fork to help crumble it into pea-sized pieces, if desired. I like to use my hands to smash it together and break off larger chunks. Sprinkle the crumb topping over the cake batter, covering it completely.
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5
Bake cake until golden, about 30-40 minutes. A toothpick inserted in the center should come out clean, indicating done-ness. Cool cake in the pan on a wire rack for 30 minutes. Generously dust the cake with powdered sugar before slicing and serving.
