In a medium saucepan, slowly bring 360ml of apple cider to the boil. Let it simmer until you have just 120ml left, it can take between 20-30 minutes. Take off the heat and keep to the side.
Preheat oven to 350°F/175°C. Grease your donut pan with butter
In a medium bowl, whisk the flour, baking powder, cinnamon, nutmeg and salt together.
In a stand mixer or with an electric whisk, cream the butter and sugars together until fluffy, about 4 minutes. Add the eggs one at a time, making sure it is fully incorporated. Add the vanilla extract and beat until combined.
With the mixer on low, add in the dry mixture. Then slowly pour in the apple cider while the mixer is still going.
Pipe or spoon the batter into the donut pan, filling it ⅔ of the way and then use a spoon to even the surface. Bake for 12-15 minutes, until it is golden brown.
Coating
While the donuts are baking, whisk the sugars and spices together in a small bowl. Melt the butter in a small bowl in the microwave, or in a small saucepan, and then add the remaining cider.
When the donuts are out of the oven but still warm, brush them with the butter cider mixture, and then cover in the sugar spice mixture.