Reduce cider: Pour apple cider into a small saucepan and simmer over medium heat until reduced by about half (10–15 minutes). Set aside to cool slightly.
Preheat oven: Preheat to 350°F (175°C). Lightly grease a donut pan.
Mix dry ingredients: In a bowl, whisk flour, baking powder, salt, cinnamon, nutmeg, and optional spices.
Mix wet ingredients: In another bowl, whisk sugars, eggs, melted butter, milk, vanilla, and reduced apple cider until smooth.
Combine: Add dry ingredients to wet ingredients and mix just until combined. Do not overmix.
Fill pan: Spoon or pipe batter into donut cavities, filling about ¾ full.
Bake: Bake 10–12 minutes, until donuts spring back lightly when touched.
Cool: Cool in pan 5 minutes, then transfer to a wire rack.
Optional topping: Brush warm donuts with melted butter and toss in cinnamon sugar.