Preheat the oven to 350 degrees F. Lightly grease muffin tin with cooking spray or oil.
In a small bowl, stir together chia seeds and 3 tablespoons of water. Set aside.
In a medium bowl, combine the milk and lemon juice. Set aside.
In a large bowl, combine the flour, cocoa powder, baking powder, baking soda and salt.
To the bowl with the milk and vinegar mixture, stir in the chia seed mixture, the maple syrup, and vanilla extract. Pour the milk mixture over the flour mixture and stir until just combined, being careful not to over mix. Fold in the walnuts, shredded zucchini, and chocolate chips (if using).
Distribute the batter equally between the muffin tin wells. Bake for 15 to 17 minutes, or until a toothpick inserted into the centre of the muffin comes out clean. Allow the muffins to cool for 5 minutes.
Run a knife around the edges of the muffins to release them from the pan and transfer them to a rack to cool completely.