Preheat oven to 350 degrees F. Spray or butter cake pan and place a round piece of parchment paper at the bottom of the pan.
In one bowl, add flour, baking powder, salt, and cinnamon. Put aside.
In a separate bowl, add butter and sugar into mixing bowl and turn electric mixer to medium for 2-3 minutes, scraping the bowl occasionally
When butter looks lighter in color, add vanilla and eggs. Mix for 2 more minutes on medium speed.
Slowly add in sour cream and milk.
Mix until combined.
Slowly add the dry ingredients, scraping the bowl occasionally so that the ingredients evenly disperse.
Mix until just combined.
In a small bowl, mix cinnamon swirl ingredients (butter, brown sugar, and cinnamon) until combined.
Pour cake batter mixture into cake pan.
Place small scoops of the cinnamon swirl mixture on top of the cake batter. Use a toothpick or something similar to swirl around.
Once the cake mixture looks ideal, place in the oven and set a timer for 35-38 minutes. I recommend checking in at 30 minutes since most ovens perform a bit differently.
Once toothpick inserted comes out with moist crumbs, take the cake out of the oven and let cool partially in pan. When still warm, but comfortable to touch, ue a knife to separate cake sides from the pan and transfer to a plate or cake board.
Slice however you like and enjoy!