Preheat oven to 350°F and grease a donut pan.
Whisk together the flour, cocoa powder, sugar, baking powder, baking soda, salt, and espresso powder.
In another bowl, whisk together the Greek yogurt, almond milk, egg, melted butter, and vanilla.
Stir in the squeezed zucchini.
Fold the wet ingredients into the dry ingredients until just combined. Fold in chocolate chips if using.
Fill donut cavities about ¾ full.
Bake for 11–13 minutes.
Cool for 5 minutes before removing from the pan.
Whisk together the glaze ingredients until smooth.
Dip cooled donuts into the glaze and allow it to set before serving.