Heat the apple cider in a small saucepan until it is reduced by 1/3, resulting in ½ cup. Set aside to cool.
Preheat the oven to 350, grease a donut pan.
Combine the flour, baking soda, seasalt, cinnamon and nutmeg in a small bowl, set aside.
In a large mixer, combine the brown sugar, sugar and butter. Whip for 3-4 mins, until the color turns light brown. Add in the eggs one at a time, mix until combined, then add in the vanilla extract.
Gradually add the dry mixture to the wet mixture in the mixing bowl at low speed. Once combined, slowly add in the room temperature apple cider until the batter is homogenous.
Fill a piping bag with the batter and pipe the batter into the donut pan to be ⅔ full.
Bake for 10-15 mins in the oven, rotating the pan halfway through.
While the donuts are baking, prepare the sugar coating by combining the sugar and cinnamon in a shallow bowl. Set aside. Melt the butter in a separate container. Set aside.
Once the Donuts are done baking, Let it cool for 5 min then transfer donuts to a wire rack.
Once the donuts are cool, brush on the melted butter and dredge in the cinnamon sugar mixture.