Preheat oven to 375 degrees F. Grease a 2 inch doughnut pan with a nonstick cooking spray and set aside.
In a large size bowl, sift together the flour, cocoa, baking powder, baking soda and salt.
In a separate bowl, beat together the butter and sugar until creamy.
Add in the eggs, vanilla and beat until well combined.
Stir in the flour mixture alternately with the milk. Stating and ending with the flour mixture.
Add in the zucchini and the nuts. Stir till combined.
Spoon the doughnut batter into a large resealable plastic bag. Snip off one corner of the bag and pipe the batter into the prepared pans, filling the doughnut rings only half way through.
Bake 6 to 8 minutes or until the tops spring back when lightly touched.
When ready, let the doughnuts rest in the pan for a few minutes then gently loosen around the edges and carefully invert onto a cooling rack.
Cool and spray the doughnut pans with nonstick spray between batches.
In a large bowl, whisk together the icing sugar, cocoa powder, corn syrup and whole milk.
To coat the doughnuts with the chocolate glaze you can either dunk them in the glaze, letting any excess drip off or simply spoon the chocolate glaze over the doughnuts.
Place the doughnuts on a wire rack with a cookie sheet underneath to catch any drippings.
Top with the sprinkles of your choice.