Preheat the oven to 350°F (175°C) and grease a loaf pan well, dusting with flour.

In a large mixing bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg for 30 seconds.

In a separate medium bowl, whisk together the milk, sour cream, egg, vegetable oil, and vanilla until smooth.

Make a well in the center of the dry ingredients, pour in the wet mixture, and gently fold until just combined with a few lumps remaining.

Pour the batter into the prepared loaf pan and spread it evenly.

In a small bowl, mix the melted butter, sugar, and cinnamon for the topping, then drizzle or spoon it over the unbaked batter.

Bake for 45 to 55 minutes, checking for doneness with a toothpick inserted into the center.

Let cool in the pan for 10 minutes before transferring to a wire rack to cool completely.