Streusel Topping and Filling:

1/3 cup light brown sugar (packed)
2 tbsp. sugar
1 1/2 tsp. cinnamon
1/2 cup cake flour
4 tbsp. unsalted butter
1/2 tsp. vanilla

Combine all ingredients and mix with fingers until it is crumbly. Set aside.

Batter:

2/3 cup sour cream
1 1/2 tsp. vanilla
2 cups cake flour
1 cup sugar
1/2 tsp. baking powder
1/2 tsp. baking soda
1/4 tsp. salt
12 tbsp. unsalted butter (soft)

Combine egg yolk, sour cream, and vanilla on low speed of mixer.

In separate large bowl combine all dry ingredients. Mix on low speed to blend. Add butter; mix on medium speed until moist. Add sour cream mixture and continue on medium speed for at least 2 minutes after totally incorporated.

In a greased and floured 8-9 inch spring form pan, place half the batter on bottom and spread around. Crumble half brown sugar mixture on batter. Pour remaining batter on top and spread to edges of pan. Place remaining brown sugar mixture on top and spread to edges.

Bake for 55-65 minutes or until toothpick comes out clean.