Preheat the Oven: Start by preheating your oven to 350°F (175°C). Line a muffin tin with paper muffin liners or lightly grease.
Prepare the Zucchini: Grate your zucchini using a box grater. You don’t need to peel it, as the skin adds extra nutrients and color to the muffins. Once grated, lightly press the zucchini with a paper towel to remove some of the excess moisture. Set it aside.
Mix the Dry Ingredients: In a medium-sized bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. This helps ensure that the dry ingredients are evenly distributed.
Combine the Wet Ingredients: In a large mixing bowl, cream together the butter, sourdough discard, oil of choice, sugar, egg yolks, milk, and vanilla extract, and granulated sugar. Mix until the ingredients are well incorporated and smooth.
Fold in the Zucchini: Gently fold the grated zucchini into the wet mixture. The zucchini will add moisture and texture to the batter.
Add the Dry Ingredients: Slowly incorporate the dry ingredients into the wet mixture, stirring just until combined. Be careful not to overmix, as this can lead to dense muffins.
Stir in the Chocolate Chips: Finally, fold in the chocolate chips or chunks. These will melt during baking, creating pockets of gooey chocolate throughout the muffins.
Portion the Batter: Using a spoon or ice cream scoop, divide the batter evenly among the muffin cups, filling each about two-thirds full.
Bake the Muffins: Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs. The muffins should be slightly domed and have a rich, dark color.
Cool and Enjoy: Let the muffins cool in the tin for about 5 minutes, then transfer them to a wire rack to cool completely. These muffins are delicious warm or at room temperature.