WHAT YOU NEED

For the cake
1 cup unsalted butter, softened
2 cups granulated sugar
1 cup packed dark brown sugar
4 large eggs
2 cups sour cream
2 tsp vanilla extract
4 cups all-purpose flour
4 tsp baking powder
1 tsp kosher salt
2 tsp cinnamon

For the cinnamon sugar swirl
6 tbsp unsalted butter, melted
⅓ cup granulated sugar
⅓ cup packed dark brown sugar
1 tsp cinnamon

For the streusel
¼ cup unsalted butter, melted
⅓ cup granulated sugar
⅓ cup packed dark brown sugar
⅓ cup all-purpose flour
2 tsp cinnamon

To make the cake
Preheat the oven to 175°C. Generously butter a 9-inch tube pan with a removable bottom. In the bowl of a stand mixer, beat the 1 cup butter, 2 cups granulated sugar, and 1 cup dark brown sugar until smooth. Add the eggs, sour cream, and vanilla, and beat until well combined. In a separate bowl, whisk together the 4 cups flour, baking powder, salt, and 2 tsp cinnamon. Add the flour mixture to the mixer and beat on low speed until combined.

To make the cinnamon sugar swirl:
In a small bowl, stir together all the cinnamon sugar swirl ingredients with a fork until well combined.
Evenly spread a third of the cake batter in the bottom of the prepared tube pan. Dollop half of the cinnamon sugar swirl over the batter in the pan and spread into a thin layer. Place half of the remaining batter evenly over the swirl layer, and then dollop and spread the remaining cinnamon sugar swirl over this layer of batter. Evenly spread the remaining batter over the top.

To make the streusel:
In a medium mixing bowl, stir all the streusel ingredients together with a fork until a crumbly mixture forms. Sprinkle over the top of the cake batter.
Bake for 85 to 90 mins, rotating occasionally, until a long toothpick or cake tester inserted in the center of the cake comes out clean. Let cool completely before removing the sides of the pan and serving. If the streusel starts to turn dark brown before the cake is finished, tent the cake with foil until it is baked all the way through.