In a pot of boiling water, cook the potatoes until soft (about 20 minutes). Press them through a potato ricer to mash smoothly. It should yield about 1 1/3 cups of mashed potatoes. Set aside to cool.
Preheat the oven to 350 degrees and grease a donut pan with coconut oil.
Stir together the dry ingredients in a small bowl (flour, cocoa powder, baking powder, baking soda, salt). Set aside.
In the bowl of a stand mixer, mix together the egg, maple syrup, coconut milk, coconut oil, and vanilla extract. Add the dry ingredients and mix to combine. Then stir in the mashed potatoes and mix well. The batter will be thick but somewhat pourable.
Pour the batter into the donut pan. Fill each donut circle almost to the top, but don’t overfill or the hole will close up during baking. Smoothe out the batter for each donut with a small spatula. (This recipe makes 18 donuts, so you’ll need to bake them in batches.)
Bake the donuts for 13 to 15 minutes, or until they’re set and a toothpick inserted in one of the donuts comes out clean.
While the donuts are baking, make the glaze. Melt the coconut oil in a small pot over low heat, then add the maple syrup, coconut milk, and vanilla extract to the pot. Warm the ingredients briefly over low heat, then remove the pot from the stove.
Let the donuts cool in the pan for 10 minutes, then carefully remove them to a cooling rack. Pour the glaze over them while they’re still warm. Sprinkle with a tiny bit of fine sea salt if desired. (This gives a great flavor contrast.)