In another bowl, combine Flour, Baking Powder, Baking Soda, and Salt.
In the bowl of a stand mixer, cream 1/2 cup Butter while slowly adding in the 2/3 cup Sugar.
Add the Eggs one at a time, beating well after each addition, and add in the Vanilla Extract.
With the mixer on low, begin adding in the flour and the sour cream, beginning and ending with the sour cream. Mix until smooth.
Spread the batter into the 9" pan. Sprinkle with the streusel topping.
Bake 325* for 45-55 minutes or until golden.
Cool for 10 minutes in the pan and then remove from the pan and let cool on a cooling rack right-side up for two hours. Note: I rushed this process. After I made the filling, I let that cool until it wasn't warm, and then I went ahead and put the cake together. For instances where looks super matter, wait the two hours so the filling won't get melty.