Sour Cream Crumb Cake

2 1/2 cups cake flour (this is important it makes a lot of difference)
2 teaspoons baking powder
1/4 teaspoon baking soda
3/4 teaspoon salt (can be reduced or omitted)
1 cup unsalted butter or margarine, softened
1 1/2 cups sugar
3 large eggs, room temperature
2 teaspoons vanilla extract
1 cup sour cream or yogurt, room temperature

Streusel:
2/3 cup all-purpose or whole wheat flour
1/2 cup packed brown sugar
pinch of salt
1/4 cup unsalted butter or margarine, softened

Preheat oven to 350. Butter and flour a 13 x9 baking pan. Whisk together cake flour, baking powder, baking soda and salt in a small bowl; set aside. Beat butter and sugar in a large bowl until light and fluffy, 2 to 3 minutes. Add eggs one at a time, beating until just mixed. Add vanilla extract. Add flour mixture alternately with sour cream, mixing until just combined, ending with flour. Spread batter evenly in pan.

For streusel, mix flour, sugar and salt in a small bowl. Work butter with fingers into mixture until it begins to clump. Sprinkle streusel evenly over the batter in the pan -- it will be a thick layer.

Bake cake in the middle of oven for 35-40 minutes or until it tests clean. Cool on a wire rack for 15-2o minutes. Cut into squares and serve.