Simmer apple cider in a small saucepan over medium-low heat for 15 to 20 minutes until reduced to exactly 120 ml (120 ml). Cool completely to room temperature.

Preheat oven to 175 °C (177 °C) and thoroughly grease two 6-cavity standard donut pans with nonstick cooking spray.

In a large mixing bowl, whisk together all-purpose flour, baking soda, baking powder, ground cinnamon, nutmeg, and sea salt.

In a medium bowl, whisk melted butter, egg, light brown sugar, granulated sugar, buttermilk, vanilla extract, and cooled cider reduction until smooth.

Pour the wet ingredients into the dry ingredients and fold gently with a silicone spatula just until combined, taking care not to overmix.

Transfer batter into a piping bag or zip-top bag with the corner snipped, and pipe evenly into donut cavities until about three-quarters full.

Bake at 175 °C (177 °C) for 10 to 11 minutes until the edges are lightly golden and tops spring back when gently pressed.

Cool in pans for 5 minutes, then turn out onto a wire rack. In two separate shallow dishes, place melted butter and combined cinnamon sugar. Brush or dip each warm donut in butter, then dredge thoroughly in cinnamon sugar to coat.