Make vegan buttermilk:

Prepare vegan buttermilk by combining ¾ cup non-dairy milk with 2 tbsp apple cider vinegar; allow to rest for 5 minutes.

Whisk dry ingredients:

In a large bowl, whisk 2 cups all-purpose flour, ½ cup granulated sugar, 2 tsp baking powder, ½ tsp baking soda, ½ tsp salt, and 1 tsp ground cinnamon (if desired).

Mix wet into dry:

Pour in the vegan buttermilk, ¼ cup unsweetened applesauce, 3 tbsp melted coconut oil, and 1 tsp vanilla extract. Mix until just incorporated; avoid overmixing.

Fill donut pan:

Lightly grease a donut pan. Fill each cavity about ¾ full with batter.

Bake donuts:

Bake at 350°F (175°C) for 12-15 minutes, until a toothpick inserted into a donut emerges clean. Let cool in the pan for 5 minutes, then transfer to a wire rack to cool fully.

Prepare glaze:

For the glaze: in a small bowl, whisk 1 cup powdered sugar, 2-3 tbsp non-dairy milk, and ½ tsp vanilla extract until smooth. Dip cooled donuts into the glaze and allow to set before serving.