Shred the zucchini and spread on a paper towel. Allow to dry for 5-10 minutes.
In a large bowl, combine flour, sugar, salt, baking powder and cocoa. Stir in zucchini and chocolate chips until well coated by flour mixture. Add buttermilk, eggs and butter. Stir until just combined.
Grease donut pan with spray oil. Spoon donut mixture into donut pan, filling ¾ full. Bake for 7-9 minutes, until tops spring back when gently pressed. Cool in pan for 5 minutes; transfer to rack and cool completely.
Melt chocolate chips in a microwave-safe bowl in 10 second increments, stirring in between. Scoop melted chocolate into a zip-lock bag, snip the corner, and drizzle over cooled donuts.
To convert these into cupcakes, increase baking time to 15-18 minutes (for medium sized muffin trays).