Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan, or line it with parchment paper, leaving an overhang on the longer sides for easy removal.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy.
Step 4: Beat in the eggs, one at a time, ensuring each egg is fully incorporated before adding the next. Scrape down the sides of the bowl as needed.
Step 5: Stir in the vanilla extract and sour cream until just combined.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix, as this can lead to a tough bread. A few streaks of flour are acceptable.
Step 7: Pour the batter into the prepared loaf pan and smooth the top. Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 8: While the bread bakes, prepare the cinnamon sugar topping. In a small bowl, combine the granulated sugar and ground cinnamon. Set aside.
Step 9: Once the bread is done, remove it from the oven and let it cool in the pan for 10-15 minutes.
Step 10: Carefully invert the bread onto a wire rack. While still warm, brush the entire surface generously with melted unsalted butter. Immediately sprinkle the cinnamon sugar mixture over the buttered bread, ensuring an even coating. Let the bread cool completely before slicing and serving, or enjoy it slightly warm for an extra comforting treat!