Reduce the apple cider. Add the cup of apple cider to a small pot and simmer over low heat until reduced to 1/3 cup. Check it at 10 minutes and then every 5 minutes after that until you're left with the desired amount. Set aside and let it cool while you prepare the rest of the ingredients.
Preheat your oven to 350degrees F. Grease a donut pan and set aside.
Add the flour, baking soda, baking powder, cinnamon, nutmeg and salt in a large bowl. Whisk and set aside.
In another bowl, whisk the melted butter, egg, brown sugar, granulated sugar, milk and vanilla until combined. Pour the mixture into the dry ingredients and whisk. Then add in the apple cider and whisk until smooth and no lumps remain.
Spoon or pipe into prepared donut pan. Fill only about halfway because they really pop up and grow!
Bake for about 10 minutes or until toothpick inserted comes out clean. The tops will be lightly browned and when you poke one, it'll bounce back up. Grease the pan again and bake that last donut!
Allow the donuts to cool in the pan for a minute or two and then flip on a wire rack to cool. While they are cooling, prepare the topping.