Cream 6 tablespoons of butter and 3/4 cup sugar until light and fluffy. Slowly beat in the eggs, one at a time, beating well after each addition. Beat in the vanilla extract and sour cream until blended.
In another bowl, combine 1 1/2 cups flour, baking powder, soda, 1/2 teaspoon of salt. Slowly beat the flour mixture into the batter until blended. Pour the batter into a well greased, 9 inch round pan.
Next, combine 8 tablespoons of butter with 1/4 cup brown sugar, 1/4 cup white sugar, 3/4 cup flour, cinnamon, and 1/8 teaspoon salt. Mix together, then crumble the topping with your fingers on the top of your cake.
Bake for 45 minutes, or until tester inserted to the center comes out clean.