Bring a pot of water to a boil, add the fresh spinach for 30–45 seconds until wilted, drain and rinse with cold water, then wring the spinach tightly in a clean towel to remove as much liquid as possible.

Transfer the squeezed spinach to a small blender with 2 tablespoons warm milk and puree until very smooth; set aside.

Preheat the oven to 375°F and line a 12-cup muffin tin with liners or grease the cups lightly; in a large bowl whisk together the flour, cocoa, baking powder, baking soda, salt, and instant coffee.

In a separate bowl whisk the eggs with both sugars until smooth, stir in the melted butter and 1/2 cup milk, then fold in the spinach puree until evenly combined.

Make a well in the dry ingredients and pour in the wet mixture; fold gently with a spatula just until mostly combined and no large streaks of flour remain, then fold in chocolate chips if using.

Scoop batter into the prepared muffin tin using a 1/4-cup measure, filling each cup nearly full for domed tops; sprinkle coarse sugar if desired.

Bake at 375°F for 18–20 minutes until tops spring back when pressed and a toothpick inserted into the center comes out with a few moist crumbs; cool in the tin 5 minutes, then transfer to a rack to cool completely.