Preheat the oven to 350°F. Grease a 10" square pan with butter. Combine the ingredients for the cinnamon streusel layer in a medium size bowl and stir with a fork until clumps form. Set aside.
In a large mixing bowl, beat the butter and the sugar together until light and fluffy, about 3-4 minutes. Stir in the eggs and vanilla extract. Add the flour, baking powder, and cinnamon and stir to combine. Mix in the sour cream.
Pour ½ of the batter into the prepared pan. Use a spatula to spread a thin layer across the bottom of the pan. Sprinkle half the cinnamon streusel over the batter and spoon the remaining batter over the streusel layer. Sprinkle the remaining topping over the batter in the pan.
Bake for 40-42 minutes, until lightly browned and a toothpick comes out with moist crumbs. Let cool completely before slicing.
Notes
COOK’S NOTE: This cake freezes well. Wrap slices tightly in an airtight container or zip-close bags. Thaw at room temperature or reheat from frozen in the microwave.