Step 1: Prepare the Streusel Topping
In a small bowl, combine the flour, brown sugar, cinnamon, and salt.
Cut in the cold butter with a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Set aside.

Step 2: Make the Cinnamon Swirl
In a small bowl, combine the brown sugar and cinnamon. Set aside.

Step 3: Prepare the Cake Batter
Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round or square cake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Mix in the vanilla extract.
Alternately add the dry ingredients and the sour cream to the butter mixture, starting and ending with the dry ingredients. Mix until just combined.

Step 4: Assemble the Cake
Spread half of the batter into the prepared cake pan.
Sprinkle half of the cinnamon-sugar mixture over the batter.
Add the remaining batter on top, smoothing it out with a spatula.
Sprinkle the remaining cinnamon-sugar mixture on top, then gently swirl it into the batter with a knife or toothpick.
Top with the streusel topping.

Step 5: Bake the Cake
Bake the cake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Step 6: Serve
Serve warm or at room temperature with your favorite coffee or tea. Enjoy!