Preheat oven to 325. Coat a 9x5 loaf pan with cooking spray. For the batter, in an electric mixer, cream together butter and sugar on medium speed until well blended. Add the eggs one at a time, beating well after each addition. Add the vanilla and sour cream and mix on low sped for 30 seconds. Using a wooden spoon, stir in flour, baking powder, baking soda and salt just until moistened and the flour has disappeared.
For the cinnamon swirl, in a small bowl, stir together sugar and cinnamon. Spoon half of the batter into the prepared loaf pan and sprinkle with half of the cinnamon sugar mixture. Pour remaining batter on top. Sprinkle with remaining cinnamon sugar and drizzle with melted butter. Bake for 55 minutes. Let cool in pan for 10 minutes. Serve warm.