In the bowl of your stand mixer fitted with the paddle attachment add in all of your dry ingredients; flour, sugar, baking powder, baking soda, and salt. Give it a mix on low just to combine the ingredients evenly.
Chop your butter into chunks (the butter should still be cold but lightly softened, not rock hard, we do not want it to be creamy).
Add the chopped butter and ½ cup of sour cream in with your dry ingredients. Mix on medium speed until the butter and sour cream moisten the flour and come together into a ball. Do not over-mix.
In a medium-sized bowl whisk together eggs, 1 cup of sour cream, and vanilla until nicely blended.
Add the egg mixture into the flour mixture in 3 additions, beating for about 20 seconds after each addition. Once all the egg mixture is added, beat for another minute until the batter is light and fluffy.
Spread 2 cups of batter into the bottom of your prepared cake pan. Sprinkle 1 cup of streusel evenly over the top (break the larger pieces of streusel apart as you sprinkle them onto your batter).
Add another 2 cups of batter on top and spread it around using a spatula. Make it as even as you can, it does not need to be perfect.
Add another 1 cup of streusel on top.
Add the rest of the batter and spread evenly.
Sprinkle another 2 cups of streusel on top of the cake. You wil have about 1 cup of streusel left, reserve it (we will use it later).
First, we are going to bake the cake for 25 minutes in our preheated oven. After 25 minutes quickly remove the cake and close the oven door. Sprinkle the reserved streusel over the cake focusing on the areas where the cake has come up through the streusel. Quickly place back in the oven and bake for another 20-25 minutes or until a a wooden toothpick inserted into the center of the cake comes out clean.
Let the cake cool on a wire rack for 20 minutes while you make the icing.
