Ingredients:

170g butter, room temp (1.5 sticks) 
225g sour cream, room temp (3/4 cup)
300g sugar (1.5 cups) 
3 eggs, room temp
1 tsp vanilla
1/4 tsp. Salt
1/2 tsp. Baking soda
2 tsp. Baking powder
275g plain flour (2 1/4 cups) 

3 Tbls. Brown sugar (50g)
1 Tbls cinnamon (10g) 
1 Tbls milk or cream 

Method:

Preheat oven to 170C (325F). Butter a bundt pan or loaf pan and set aside. 

Mix together flour, salt, baking powder and baking soda, set aside.  Also, mix together the brown sugar, cinnamon and milk (or cream) in a small bowl and set aside.

In an electric mixer fitted with a paddle attachment, cream together butter and sugar until light and fluffy.  Add in vanilla.  Crack your eggs into a bowl or cup and whisk them together.  With the machine running, slowly pour the eggs into the creamed butter and sugar.  Just a little at a time. Once fully incorporated but not curdled, remove the bowl from the mixer and switch to a rubber spatula to mix by hand.  Add half the sour cream and fold it in.  Then add half the dry ingredients and fold them in. Next the remaining sour cream followed by the remaining dry ingredients.  This is a thick batter.  Spoon out 1/3 of the batter and mix it together with the brown sugar mix.  Spread 1/2 of the remaining batter into the tin, followed by a layer of the brown sugar batter and then top with the remaining plain batter.  Spread it evenly into the tin.  Then use a chopstick or skewer to create a Swirl pattern.  Bake for 40-60 minutes until a cake tester comes out clean, start checking at 40 minutes to avoid overcooking.  Allow to cool and flip out.  Serve with coffee or tea.