Pour 1 cup of apple cider into a small saucepan. Boil the mixture, then lower the heat.Let it simmer until it reduces to about ⅓ cup. Stir occasionally for 12 to 15 minutes. Let cool slightly.
Preheat oven to 350°F (175°C). Lightly grease a donut pan with cooking oil spray.
In a medium bowl, whisk together almond flour, tapioca flour, baking soda, cinnamon, and salt.
In a large bowl, whisk together the eggs, sugar, melted butter, almond milk, vanilla, and reduced apple cider until smooth.
Add dry ingredients to wet and stir until just combined (do not overmix). Batter will be thick.
Spoon or pipe batter evenly into donut pan cavities, filling each about ¾ full.
Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes, then transfer to a wire rack.
In one bowl, mix sugar and cinnamon. Place melted butter in another small bowl. While still warm, brush or dip donuts lightly in melted butter, then roll in cinnamon sugar mixture. Enjoy!