Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan. Or, line it with parchment if you're feeling fancy.
In a mixing bowl, cream together the butter and granulated sugar until light and fluffy. This might take a couple of songs if you're using a hand mixer.
Beat in the eggs, one at a time, then add the sour cream and vanilla. Mix until it’s all cozy and blended.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry mix to the wet mix in batches, stirring until just combined. No need to overthink it.
In a small bowl, stir together the brown sugar and cinnamon for the swirl.
Spread half of the batter into the prepared pan. Sprinkle with half of the cinnamon-sugar mix. Dollop the remaining batter on top and gently spread. Finish with the rest of the cinnamon-sugar.
Bake for about 30 minutes, or until a toothpick comes out clean. Let it cool slightly before serving. Or don’t. Warm is heaven.