Reduce the cider: In a small saucepan, simmer apple cider over medium heat until it reduces to about 3/4 cup. This will take about 15 - 20 minutes and makes the flavor much stronger. Let it cool.
Preheat oven to 350 degrees F. Grease a bundt pan (or loaf pan) well.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. Add eggs one at a time, mixing well after each. Stir in vanilla and cooled cider reduction.
In a separate bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Add dry ingredients to wet ingredients in batches, alternating with sour cream. Mix just until combined. Overmixing can make the cake dense.
Pour batter into the prepared pan and bake for 35-45 minutes (bundt) or until a toothpick comes out clean. Adjust time if using a loaf pan.
Cool for 15 minutes, then carefully turn out onto a cooling rack.
Cinnamon sugar topping: While the cake is still warm, brush it with melted butter, then sprinkle generously with cinnamon sugar.
