Preheat your oven to 350ºF (175ºC).

Spray your donut pan generously with cooking spray to prevent sticking.

In a large bowl, whisk together the flour, flaxseed meal, baking soda, cream of tartar, baking powder, salt, and sugar until well combined.

In another small bowl, whisk together the melted butter, vanilla, egg yolks, and coconut milk.

Create a well in the center of the dry ingredients and pour in the wet mixture. Gently mix until just combined.

In another small bowl, whisk the egg whites for 1-2 minutes until slightly thickened.

Fold the whipped egg whites gently into the batter until just combined; do not overmix.

Transfer the batter to a Ziploc bag and cut a corner off for piping.

Pipe the batter into the donut pan wells, filling them about 3/4 full.

Bake for 8-10 minutes, or until a toothpick inserted comes out clean and the donuts are lightly golden.

Allow the doughnuts to cool completely in the pan before glazing.

In a bowl, mix the powdered sugar with 1 to 2 tablespoons of milk until smooth and adjust the consistency as desired.

Dip the cooled doughnuts into the icing and place them on a wire rack to allow excess glaze to drip off and set.