Preheat oven to 350F (175C) and grease and flour a 9x9” (23x23cm) pan or spray with baking spray.
In a large bowl (or the bowl of a stand mixer), stir together flour, sugars, baking powder, salt, and baking soda until completely combined.
Using an electric mixer (or paddle attachment of mixer), add butter just 1 tablespoon at a time, not adding the next tablespoon until the previous has been completely incorporated. Once all butter has been added, mixture will resemble coarse sandy crumbs.
In a separate bowl or large measuring cup, whisk together sour cream, egg, egg yolk and vanilla extract until well-combined.
Turn mixer speed to low and slowly add sour cream mixture into batter, stirring until completely combined. Be sure to pause and scrape the sides and bottom of the bowl as needed. Once finished, the batter should be smooth and silky.
Spread into prepared pan and set aside while you prepare the crumb topping.
In a medium-sized bowl, whisk together flour, sugars, salt, and cinnamon.
Drizzle melted butter over the flour mixture and use a fork to toss the ingredients together until you have a clumpy crumble topping.
Scatter crumb topping evenly over the batter in your pan.
Transfer to 350F (175C) oven and bake for 45-50 minutes or a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow to cool before cutting and serving.