Crumb Topping
1 1/2 Cups Cake Flour
2/3 Cup Dark Brown Sugar
2/3 Cup Sugar
1 1/2 Tsp Cinnamon
1/4 Tsp. Salt
!/2 Cup Melted & Cooled Butter
Cake
3 Cups Cake Flour
1 1/2 Cups Sugar
1 1/2 Tsp. Baking Powder
1 1/2 Tsp. Baking Soda
1 1/2 Cups Sour Cream
1/4 Cup Milk
3 Room Temperature Eggs
2 Room Temperature Egg Yolks
3/4 Cup Melted & Cooled Butter
1 Tbsp. Vanilla
3/4 Tsp. Salt
Crumb Topping
Whisk together flour, sugars, cinnamon, and salt in medium bowl. Add melted butter and mix with hands or flexile spatula until crumbly and holds together when squeezed.
Cake
Heat oven to 350° and coat 9 X 13 inch cake pan with cooking spray. Line bottom and sides with parchment paper long enough to extend 2 ” to form a sling.
Whisk cake flour, sugar, baking powder and soda together.
Whisk together sour cream, milk, eggs, egg yolks, butter, vanilla, and salt in another bowl. Pour into flour mixture and stir with flexible spatula until just combined. Some lumps are OK. Pour into the cake pan and smooth out the top.
Using your hands crumble the topping into rough pieces, no larger than a blueberry, spread evenly over the top of the batter. Gently press crumbs into the batter.
Bake cake 40-60 minutes, rotating cake pan halfway through. If cake tester comes out completely clean when placed in center of cake, it is done. Let cool for one hour after removing from oven. You can lift the cake out of the pan by grabbing the parchment paper. Then cut the cake and serve.