Heat the apple cider in a large saucepan over medium high heat until reduced to 1/3 cup, should take about 10 minutes.
Cream the butter: In a large bowl using a hand-held or standing mixer fitted with a whisk attachment, beat together the sugar and the butter until mixture is pale, about 3 minutes. Add the eggs and continue beating for another 2 minutes until light and fluffy.
Combine wet ingredients: Pour the buttermilk, reduced cider, and vanilla into sugar/butter/egg mixture. Mix well, and don't worry if the mixture looks a bit curdled; it'll smooth itself out.
Combine the dry ingredients: In a medium-size bowl, whisk together flour, salt, baking powder, baking soda, cinnamon, and nutmeg; set aside.Combine the wet and dry ingredients: Add the flour mixture to the bowl with the wet ingredients and combine gently just until fully moistened.
Form the dough: Transfer the dough to a lightly floured surface and knead it a few times until it comes together; form it into a ball. Roll out the dough so that it's 1/2-inch in thickness. Cut out rounds with a 3-inch cutter donut cutter. Don't discard the donut holes, fry those too.
Make cinnamon sugar: In a shallow bowl combine the 1 1/2 cups granulated sugar with the 3 tbsp of ground cinnamon. Set aside.
Heat vegetable oil in a large stockpot or Dutch oven over medium high heat until a deep-fry thermometer registers 375 degrees F.
Fry the donuts: Working in batches, add donuts to the Dutch oven and cook until evenly golden and crispy, 1 minute per side. Transfer to a paper towel-lined plate for 30 seconds; then roll warm donuts in cinnamon sugar mixture to coat.