Preheat the oven to 375 degrees F. Grease a standard muffin pan with nonstick spray.
Add the spinach, yogurt, maple syrup, eggs, butter, and vanilla to a blender. Blend very smooth.
In a medium bowl, stir together the flour, cocoa powder, chocolate chips, baking powder, baking soda, and salt.
Add the blended spinach mixture.
Gently stir or whisk together.
Divide batter among the prepared muffin tin, using a heaping ¼ cup batter in each cup. Sprinkle additional chocolate chips on top, if desired.
Bake for 14-16 minutes, or until the edges are lightly golden brown and a cake tester inserted into the center comes out cleanly. Remove from oven, then transfer muffins to a wire rack to cool fully.
Notes
Once cooled, store in an airtight container for up to 3 days at room temperature. Store in the fridge in an airtight container for up to 5 days. Warm slightly to serve from the fridge (they firm up a bit when they’re cold so warming slightly returns them to their softer texture).
You can also freeze the muffins in a freezer bag with as much air removed as possible for up to 3 months. Thaw at room temperature or in 15-second increments in the microwave.
To make as mini muffins, divide batter in a greased 24-cup mini muffin pan and bake for 12-14 minutes.
Dairy-free: Use a nondairy Greek-style yogurt, like a plant-based option from Siggi's, and a neutral oil, like canola, in place of the butter.
Gluten-free: Substitute cup-for-cup gluten-free flour in place of the wheat flours.
Egg-free: Omit the eggs. Reduce the yogurt to ¾ cup. Add ¼ cup additional milk. Proceed with the recipe.
Feel free to replace the chocolate chips with diced berries or other fruit, or to increase the amount of chocolate chips to 1 cup. Up to you!