In a small saucepan over medium-low heat, gently reduce the apple cider to about ¼ cup (4 tablespoons), about 30-40 minutes. Set aside to cool.
Make the cinnamon sugar
Combine the sugar and cinnamon together. Set aside.
Make the apple cider donut balls
In a large bowl, mix together the flour, salt, baking powder, baking soda, cinnamon and nutmeg. Set aside.
Using a stand mixer with the paddle attachment on medium speed, beat the sugar and butter until pale and fluffy, about 5 minutes. Add in the eggs one at a time and beat until well mixed, about 2 minute. Add in the buttermilk, apple cider syrup and vanilla and beat on low speed until well mixed, about 2 minutes.
Continue beating on low speed, and add in the flour mixture in 3 additions. Beat until the dough comes together. If you lightly poke the dough, the dough should hold an indent, but it will be sticky. If it is too sticky to handle, add more flour about 2 tablespoons at a time.
Place the bowl in the freezer until the dough stiffens, about 20 minutes.
In a deep fryer, heat the oil to 350°F/180°C. Using a 2 tablespoon cookie scoop, make as many donut balls as you can fit in your deep fryer (I made 4 at a time), and set them on a small piece of parchment paper. Using your fingers, smooth out any rough edges of the dough.
Carefully add the donut balls to the oil, and fry until golden brown, about 1 minute. Turn the donut balls over and fry until the other side is golden, about 1 minute. You may need to use a spatula to hold down the donut balls in the oil to get an even browning since they tend to bounce around and rotate on their own. Once the donuts are fried, transfer them to a paper towel and drain. Allow to cool slightly.
In the meantime, prepare and put the next batch of donuts into the fryer.
As the next batch is in the deep fryer, toss the fried donuts in the bowl of cinnamon sugar and serve immediately.