Make the Donuts: In a saucepan, simmer the apple cider over medium heat for 10-12 minutes, reducing it by half. Set aside to cool. In a large bowl, whisk together the sourdough starter, sugar, melted butter, eggs, vanilla extract, cinnamon, nutmeg, and clove. Stir in the cooled apple cider reduction. In a separate bowl, combine the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture and stir until just combined. The dough should be thick but not too stiff.

1. Shape the Donuts: Roll the dough out onto a floured surface to about 1/2 inch thickness. Use a donut cutter or two round cookie cutters to cut out donut shapes. Gather any excess dough and reroll to make additional donuts.
2. Fry the Donuts: Heat oil in a deep skillet or pot to 350°F (175°C). Carefully fry the donuts for 2-3 minutes per side, until golden brown and cooked through. Use a slotted spoon to remove the donuts and transfer them to a paper towel-lined plate to drain excess oil.
3. Coat in Cinnamon Sugar: While the donuts are still warm, roll them in the cinnamon sugar mixture until evenly coated.
4. Make the Spiced Plum Curd: In a medium saucepan, combine the chopped plums, sugar, lemon juice, cinnamon, ginger, and cloves. Cook over medium heat until the plums break down and the mixture thickens, about 10-12 minutes. In a heatproof bowl, whisk the egg yolks. Gradually add a spoonful of the hot plum mixture to the yolks to temper them, then whisk the yolk mixture back into the saucepan. Cook over low heat until thickened, about 5-7 minutes. Remove from heat and whisk in butter, one piece at a time. Strain the curd to remove solids and let it cool to room temperature.
5. Serve: Serve the warm, cinnamon-sugar-coated donuts with a generous dollop of spiced plum curd on top or on the side for dipping.