Simmer the apple cider in a saucepan over medium heat until it reduces from 2 cups to about 1 cup, usually 15–20 minutes. Cool the reduced cider to lukewarm.

Heat the oven to 350°F (175°C). Thoroughly grease and flour a 10- or 12-cup Bundt pan, making sure to coat every ridge and the center tube.

Whisk the flour, baking powder, baking soda, cinnamon, nutmeg, and salt together in a medium bowl. Keep the dry mixture nearby while you prepare the butter mixture.

Beat the softened butter, granulated sugar, and brown sugar for 3–4 minutes, until light and fluffy. Add the eggs one at a time, scraping the bowl as needed, then mix in the vanilla.

Add the dry ingredients in three additions, alternating with the reduced cider in two additions. Mix only until the batter is combined, because overmixing can make the cake dense.

Spread the batter evenly in the prepared Bundt pan and smooth the top. Bake for 45–55 minutes, until a skewer inserted into the thickest part comes out clean or with a few moist crumbs.

Cool the cake in the pan for 10–15 minutes, then turn it out onto a rack. Let it cool until warm rather than hot before applying the finish.

Brush the warm cake with melted butter. Stir together the sugar, cinnamon, and optional nutmeg, then sprinkle or press the mixture over the buttered cake while the surface is still tacky.