In a bowl, sift together the flour, baking powder and baking soda. Set aside.
In another bowl, mix together the ingredients for the streusel and set aside.
Using an electric mixer (I used a stand mixer), cream the butter and sugar until light and fluffy.
Add the vanilla and eggs and mix well.
Scrape down the sides after two minutes and mix until well incorporated.
Add half of the sour cream and half of the flour mixture. Mix and then add the rest of the sour cream and flour. Mix well.
Spread the batter into the prepared cake pan and then sprinkle the streusel on top.
Bake the cake for 30 minutes. Insert a toothpick in the middle of the cake and when it comes out clean with no crumbs or wetness, then the cake is done.
Notes
The original recipe said it may take about 45 minutes to bake, depending on your oven. Mine took exactly 30 minutes at 325F.
Recipe by sprinkles and sauce at https://sprinklesandsauce.com/sour-cream-coffee-cake/