Preheat oven to 350 degrees F. Spray a 9-inch square baking dish with baking spray. Set aside.
In a medium bowl, combine streusel ingredients and mix with a fork until thick and crumbly. Set aside.
In a large mixing bowl, combine the butter and sugar for the cake layer. Mix until light and fluffy, about 3 minutes. Beat in the eggs, one at a time. Add vanilla.
Add the flour, baking powder and cinnamon to the batter. Fold in the sour cream.
Pour half the batter into the bottom of your prepared pan. Spread with an offset spatula (it will be a thin layer, don't worry). Add about half of the cinnamon streusel layer. Spoon the remaining batter over the top and finish with the remaining streusel.
Bake in preheated over for about 40 minutes, until lightly browned. Cool completely before slicing.
Store in airtight container at room temperature. To freeze, place slices in ziploc freezer bag and freeze. Thaw at room temperature and enjoy!
Notes
Spray your baking pan with non-stick baking spray first to make the cake easier to remove. Or use our homemade cake release.
Use an offset spatula to smooth the first layer of cake batter into the bottom of the pan in an even layer. It will be thin; that’s OK.
The second layer of cake batter, on top of the sprinkled streusel, can just be spooned on. It doesn’t need to be spread out.
Cool the cake completely before you slice it. It’s hard to be patient, but this allows it to set up properly.