Pre-heat oven to 350 degrees and lightly butter a 9″x13″ pan.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In the bowl of an electric mixer or in a large bowl with a hand mixer, beat butter and sugar on medium speed until light and fluffy, 2-3 minutes.
Add eggs, one at a time, mixing well after adding each. Add vanilla, flour mixture, and sour cream, mixing until just blended, taking care not to over-mix.
Spread batter evenly into prepared pan. Spread the crumb topping over the batter in the pan.
Bake for 45 to 50 minutes until golden brown, turning the pan once during baking. Cool on a wire rack.