1. Preheat oven to 350°F (175°C). Grease and flour two standard loaf pans.

2. In a large bowl, cream the butter and sugar until light and fluffy.

3. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.

4. Mix in the sour cream until just combined.

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

6. Gradually add the dry ingredients to the wet mixture and stir until fully incorporated.

7. Spoon half the batter into each loaf pan, sprinkle cinnamon sugar over, then top with remaining batter and more cinnamon sugar.

8. Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.

9. Cool in pans for 10 minutes, then transfer to a wire rack to cool completely.