Preheat the oven to 325°F (165°C). Grease a 9×5-inch loaf pan and line it with parchment paper.
Bring the apple cider to a boil in a small saucepan. Reduce for about 10 minutes, until you have ½ cup. Let it cool slightly, then stir in the sour cream and vanilla.
Let it cool slightly, pour into the mixing bowl then stir in the sour cream and vanilla.
Whisk the eggs and brown sugar until smooth and slightly frothy. Slowly mix in the melted butter.
In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, salt, cinnamon, and nutmeg.
Add the dry ingredients to the egg mixture, alternating with the cider mixture. Stir just until the batter is smooth.
Pour the batter into the prepared loaf pan and bake for 50–55 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool in the pan for 10 minutes, then transfer to a wire rack and cool for another 5 minutes.
Brush the warm loaf all over with melted butter. Combine the granulated sugar and cinnamon, then press the mixture generously over the top and sides of the loaf.
Let the coating set before slicing and serving.