Stirring occasionally, simmer the apple cider, cinnamon stick, cloves, allspice and orange slices, if using, in a small saucepan over low heat until the cider is reduced to about 1/2 cup. Start checking at 10 minutes, 15 minutes, 20 minutes, etc until you have 1/2 cup (120ml).
Remove the apple cider from the heat, and remove the spices and orange slices. Pour the cider into a measuring cup, and let it cool until warm or room temperature.
Meanwhile, whisk the flour, sugar, baking powder, salt cinnamon and nutmeg together in a large bowl. Set aside.
To the room temperature cider, whisk in the whole milk, egg, melted butter and vanilla.
Create a well in the center of the flour mixture. Pour the cider/milk mixture in the center of the well and stir using a rubber spatula just until moist.
Scoop out the dough onto a sheet pan lined with parchment paper or a non-stick mat. Give the dough a few light kneads to ensure everything is sticking together. The dough will be craggy.
Pat the dough to 3/8" - 1/2" thick and cover with plastic wrap. Refrigerate for 1 hour, or freeze for 15 minutes.
In a shallow bowl, combine 1 cup of sugar with 2 teaspoons of cinnamon, and whisk to combine. Set aside.
Meanwhile heat the vegetable oil in a large stockpot or Dutch oven over medium heat until a deep-fry thermometer registers 370-375 degrees F.
Once the dough is chilled, cut out donuts with a 2 1/2" - 3" round, biscuit or donut cutter. Use a 1 1/2" round cutter to cut out the centers.
Working in batches to not overcrowd the oil, carefully add donuts to the oil and cook until evenly golden and crispy, 1 minute per side. I use a stainless steel spider to drop in the donuts and remove them from the oil. You can also use the handle of a wooden spoon.
Transfer the donuts to a paper towel-lined cooling rack for 20 seconds; then roll warm donuts in cinnamon sugar mixture to coat. Serve warm.
The donuts can be stored for up to four or five days in an airtight container. Note that the outside will soften a bit and won't be as crispy.
Keyword apple cider donuts, donuts, doughnuts, mulled apple cider cake donuts