Preheat oven to 350°F. Spray a 9 x 13" baking pan with non stick baking spray (or coat lightly with oil).
In a medium bowl, combine all crumb topping ingredients EXCEPT butter with a fork. Add melted butter and stir until crumbles form. Press the crumbs together in your hands to form large crumbs, refrigerate while making cake.
In a large bowl, cream butter and sugar together with a mixer at medium high for 4-5 minutes. Don't skimp on time, you want this to be light and fluffy.
Add eggs one at a time, mixing just until combined after each one. Beat in vanilla and sour cream, scraping down sides as needed until combined.
Sift in flour, baking powder, baking soda and salt and mix on low until everything is just combined (do not over mix at this stage or your cake will be tough).
Spread cake batter evenly in the prepared pan.
Break down the large clumps of crumb topping into smaller nuggets as you sprinkle it evenly over the top of the cake batter.
Bake for 30 -35 minutes until toothpick inserted in the cake comes out clean and the topping looks slightly crisp.
Let cool on wire rack for 15-20 minutes top with sifted powdered sugar as desired and cut into slices.
Notes
To use a 9" springform pan instead of 9x13" pan, cut the recipe by two thirds as follows:
CRUMB TOPPING
¾ cup dark brown sugar (firmly packed)
¾ cup granulated sugar
¾ teaspoon cinnamon
¼ teaspoon salt (omit if using salted butter)
1 ⅔ cups all purpose flour
¼ teaspoon baking powder
¾ cup unsalted butter (melted)
CAKE
½ cups unsalted butter
¾ cup granulated sugar
2 large eggs
1 ½ teaspoons vanilla extract
¾ cup sour cream
1 ½ cups all purpose flour
¾ teaspoon baking powder
½ teaspoon baking soda
½ teaspoons salt
Follow directions above, baking time will be about the same.