Preheat oven to 350 degrees. Spray or butter an 8 x 8 or 9 x 9 baking dish. Set aside.
In a large mixing bowl cream together white granulated sugar and softened butter. Beat in sour cream, egg, and vanilla extract.
In a separate mixing bowl, whisk together flour, baking powder, baking soda, and salt.
Gradually add in flour mixture to the wet ingredients until all combined. Spread 1/2 (half) of the batter into prepared baking dish.
In a small bowl mix together streusel ingredients (brown sugar 1/4 cup flour, cold butter pieces and cinnamon) until a crumb mixture forms. Sprinkle 1/2 of the crumb mixture on top of the batter in baking dish evenly.
Spread remaining batter on top of the crumb mixture in baking dish. (Batter will be thick.) Then top with remaining crumb mixture.
Bake in preheated oven for 35-40 minutes until golden brown. Toothpick inserted in center should come out clean. Allow to cool for 10 minutes before serving.
