Preheat oven to 350°F (175°C). Grease an 8x8-inch baking pan.
To make the cake, cream together the butter and white sugar in a large bowl until light and fluffy. Beat in the eggs one at a time, then stir in the sour cream and vanilla.
Mix in flour, baking powder, and salt. Spread 1/2 of batter in the prepared pan.
Make the streusel: In a medium bowl mix flour, brown sugar, melted butter, and cinnamon. Sprinkle cake batter with half of the streusel. Spread second half of batter over the filling, and top with remaining streusel.
Bake about 40 minutes or until a toothpick inserted near the center comes out clean.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.