The Cinnamon Filling

1. In a small bowl, whisk together the brown sugar, cinnamon, and flour until combined. Set aside.

The Streusel Topping

1. In a medium sized bowl, whisk together the brown sugar, flour, and cinnamon until combined.
2. Add in the cold, diced butter. Using your fingers, crumble the butter into the mixture until it looks like wet sand. For larger topping pieces, squeeze the mixture with your hand and break into small chunks. Once done, set aside.

The Cake

1. Grease a 9-inch springform pan and line the bottom with parchment paper. Preheat the oven to 350°F. You should also be able to use a 8×8 inch baking pan. When lining a square pan with parchment paper, make sure some hangs over the edge so you can life the cake out of the pan when it’s done.
2. In a medium sized bowl, whisk together the flour, baking powder, and salt.
3. In the bowl of a stand mixer with the paddle attachment, beat the butter and sugars for a couple minutes until light. Add the eggs one at a time, beating on low until combined after each is added. Add the sour cream and vanilla and beat on low until combined. Scrape down the sides of the bowl.
4. Add the dry mixture to the wet and mix on low until just combined.
5. Spread half of the cake batter into the pan. Then sprinkle the cinnamon filling on top until fully covered (you may not need all of the filling). Add the other half of the batter and carefully spread on top of the filling. Sprinkle the streusel topping evenly on top.
6. Bake for 45 – 50 minutes or until a toothpick inserted in the center comes out clean.
7. Let the cake cool before removing from the springform pan.

The Vanilla Icing

1. In a small bowl, whisk together the powdered sugar, milk, and vanilla. Add in more milk or powdered sugar based on your preference for the thickness of the icing.
2. When the cake has cooled, drizzle the icing on top of the cake.