Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.

In a blender, combine the spinach with about 1/4 cup of water and blend until it forms a smooth puree.

In a large mixing bowl, whisk together the cocoa powder, sugar, baking powder, baking soda, and salt.

In a separate bowl, mix the vegetable oil, eggs, vanilla extract, and spinach puree until well combined.

Pour the wet mixture into the dry ingredients, mixing just until combined.

Gradually add the flour, folding it into the mixture, and add chocolate chips if desired.

Fill each muffin cup about two-thirds full and bake in the preheated oven for about 20 minutes, or until a toothpick inserted in the center comes out clean.