In a medium saucepan over high heat, bring apple cider to a rolling boil and cook until reduced by half (1 cup total), about 12-15 minutes. Remove from heat and cool completely
In a large bowl, whisk together flour, brown sugar, baking powder, salt, baking soda, nutmeg, cinnamon, allspice, and ground cardamom
In a separate bowl, whisk eggs, melted butter, and cooled apple cider together until well blended
Create a well in the center of the flour mixture and pour wet ingredients into well. Gradually mix together until a sticky dough forms. Cover dough and chill for one hour or until firm enough to shape
Place dough on well floured surface. Gently roll or pat dough to 1/2 inch thickness. Cut with a floured donut cutter and place donuts on parchment lined baking sheet
In a Dutch oven or electric deep fryer, heat about 3 inches of oil to 365 degrees F. Fry donuts (only 3-4 at a time) for about 1 minute per side or until golden brown and cooked through
Remove donuts and drain on paper towels or a wire rack on baking sheet. Cool slightly. Roll donuts in cinnamon sugar coating if desired; serve warm