Preheat your oven to 350°F (175°C) and prepare a 9-inch round or bundt pan by greasing and dusting it with flour.

In a mixing bowl, whisk together the all-purpose flour, baking soda, and salt until well combined.

In a large mixing bowl, beat the unsalted butter and granulated sugar together until light and fluffy, about 3-5 minutes. Add in the eggs, vanilla extract, and sour cream or buttermilk, mixing until just combined.

Gradually add the prepared dry ingredients into the wet mixture, stirring gently until just combined.

In a separate bowl, mix together the unsweetened cocoa powder, cinnamon, and additional sugar to create the swirl filling.

Pour half of the cake batter into the prepared pan, spreading it evenly. Sprinkle half of the chocolate swirl filling over the batter, then layer the remaining batter on top, followed by the rest of the filling.

Using a knife or skewer, gently swirl through the batter to combine the layers slightly without completely mixing them.

Place the pan in the preheated oven and bake for 30-40 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Once baked, remove the cake from the oven and let it cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely.