Begin by reducing the apple cider: In a small saucepan, simmer the apple cider on low heat, stirring occasionally. Continue simmering until the liquid reduces to about 1/2 cup (120ml). This process typically takes around 20 minutes, but start checking from the 10-minute mark onwards. If you notice any spices or solids on the surface, leave them be. Once reduced, allow it to cool for 10 minutes.
Preheat your oven to 350°F (177°C). Lightly coat a donut pan with non-stick spray and set aside.
Prepare the donut batter: In a large mixing bowl, combine the flour, baking soda, baking powder, cinnamon, apple pie spice, and salt. In a separate bowl, whisk together the melted butter, egg, brown sugar, granulated sugar, milk, and vanilla extract. Gradually pour this wet mixture into the bowl of dry ingredients. Add the reduced apple cider and whisk everything together until a smooth, slightly thick batter forms.
To easily transfer the batter into the donut pan, spoon it into a large zipped-top bag. Snip off one corner of the bag and pipe the batter into the donut cavities, filling each halfway.
Bake the donuts for 10-11 minutes, or until the edges and tops are a light golden brown. To check doneness, gently press the top of a donut—if it springs back, it’s ready. Let the donuts cool in the pan for 2 minutes before transferring them to a wire rack. Re-grease the pan and bake the remaining batter.
For the topping: In a medium bowl, combine the granulated sugar, cinnamon, and apple pie spice. Once the donuts are cool enough to handle, dip each one in the melted butter, making sure both sides are covered, and then generously coat them in the spiced sugar mixture.
These donuts are best enjoyed fresh. Store any leftovers in an airtight container at room temperature for up to 2 days or refrigerate for up to 5 days.