In a small saucepan, simmer the 240 g (1 cup) apple cider over medium heat until reduced to about 90 g (⅓ cup), 10–12 minutes. Set aside to cool to room temperature.

Preheat the oven to 350°F (175°C) and generously grease a donut pan.

In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, nutmeg, and salt.

In a separate bowl, whisk the sourdough discard, eggs, buttermilk, melted butter, vanilla, and the cooled reduced cider until smooth.

Pour the wet ingredients into the dry and fold together just until no streaks of flour remain - the batter will be thick. Fold in the diced apple.

Spoon the batter into a piping bag (or a zip-top bag with the corner snipped off), and pipe it into the donut cavities, filling each about ⅔ full.

Bake for 11–13 minutes, until the tops spring back when touched and a toothpick comes out clean.

Cool in the pan for 5 minutes, then turn the donuts out onto a wire rack.

While still warm, brush each donut all over with melted butter.

Stir the sugar and cinnamon together in a shallow dish, then roll each buttered donut in the mixture until fully coated. Serve warm.