Preheat oven to 350 degrees F. Line two 8 inch pans with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and cream cheese until light and fluffy and smooth. Gradually add in sugar and mix until fluffy.
Turn the mixer to low and add in eggs and egg yolk one at a time. Mix after each addition until combined.
Beat in the sour cream.
In a separate bow mix together flour and baking powder.
Slowly add the dry ingredients to the wet ingredients alternating with the milk.
In a small bow mix together the melted butter, brown sugar and cinnamon until combined
Pour 1/4 of the batter into one pan and another quarter into your other pan. Then put 1/4 of the swirl mixture into each pan. Swirl the batter and the mixture together with a knife or tooth pick. Add another 1/4 of batter to each pan and another 1/4 of the swirl mixture. Swirl again.
Bake the cakes for 25-30 minutes or until a toothpick inserted comes out clean. Let cool in pans for 15 minutes before turning out onto cooling racks.