Heat the apple cider in a small saucepan to a simmer, stirring frequently, until reduced to 1/2 cup. Transfer to a small bowl and let cool to room temperature.

Preheat the oven to 350F and grease a 9x5-inch loaf pan. Set the pan aside while you mix the batter.

Whisk together the all-purpose flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a bowl. Whisk until evenly combined with no visible clumps of spice.

Whisk the melted unsalted butter, granulated sugar, brown sugar, and large eggs in a large bowl until smooth. Stir in the vanilla extract, sour cream, and cooled reduced cider until the batter looks glossy.

Gently fold the dry ingredients into the wet mixture just until combined. Stop as soon as you don’t see dry flour to keep the loaf tender.

Pour the batter into the prepared pan and bake at 350F for 45-50 minutes, until a toothpick inserted in the center comes out clean. The loaf should be golden and spring back slightly when pressed.

Brush the warm loaf with melted butter, then sprinkle with the cinnamon-sugar topping. Let it sit briefly so the topping sets into a crackly layer before slicing.