Preheat the oven to 350 degrees F and spray a donut pan with non-stick spray.
Whisk the flour, cocoa powder, sugar, baking powder, salt in a large bowl. Pour the soy milk, melted vegan butter and vanilla into the bowl with the dry ingredients. Mix well with a large wooden spoon to combine.
Spoon the batter into the donut pan, filling about 3/4 of the way full. Be careful not to overfill, as the donuts will rise and you might lose the "hole" of your donut.
Bake for 10-12 minutes or until the donuts look set on top. Let cool for two or three minutes in the pan, then carefully transfer to a wire rack with a large piece of parchment paper beneath it (to help with the mess).
Make the Glaze: In a medium sized bowl, add powdered sugar, cocoa, vanilla and soy milk. Stir with a spoon until a smooth glaze is formed. I like it to be quite thick, but if you need to thin it out you can add a teaspoon of soy milk at a time until a desired consistency is reached.
Once the donuts have cooled completely, dip the tops into the glaze carefully, then place back on the rack over parchment paper to set. You could also spoon the glaze on if you prefer.