Preheat oven to 350°F (175°C). Grease or line a 9×5-inch loaf pan.

Mix cinnamon and sugar in a small bowl and set aside.

In a large bowl, cream together butter and sugar until fluffy.

Add eggs one at a time, then stir in vanilla.

In a separate bowl, whisk flour, baking powder, baking soda, and salt.

Add dry ingredients to the wet mixture, alternating with sour cream.

Pour half the batter into the pan. Sprinkle cinnamon sugar. Add remaining batter and top with more cinnamon sugar.

Bake for 50–60 minutes or until a toothpick comes out clean. Cool before slicing.