Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper leaving an overhang for easy removal.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and ground nutmeg until well combined.
Step 3: In a separate medium bowl, whisk together the melted unsalted butter, eggs, sour cream, and vanilla extract until smooth.
Step 4: Pour the wet ingredients into the dry ingredients. Mix with a spatula or spoon just until combined. Be careful not to overmix; a few lumps are fine.
Step 5: In a small bowl, combine the brown sugar and ground cinnamon for the swirl/topping.
Step 6: Pour half of the bread batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture evenly over the batter.
Step 7: Carefully spoon the remaining batter over the cinnamon-sugar layer, spreading it gently to cover. Top with the remaining cinnamon-sugar mixture. For a beautiful swirl, gently run a knife through the top layer of batter a few times.
Step 8: Bake for 45-55 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. The top should be golden brown.
Step 9: Let the bread cool in the pan for 10-15 minutes before carefully transferring it to a wire rack to cool completely.