Prepare the Spinach:
For Fresh Spinach (Boiling): Bring a pot of water to a boil. Add the spinach, cover, and cook for 5-6 minutes until softened. Drain and rinse with cold water to maintain its green color.
For Fresh Spinach (Steaming): Place the spinach in a steamer basket over boiling water. Cover and steam for 3-5 minutes until wilted. Drain and rinse with cold water.
For Frozen Spinach: Thaw the spinach and squeeze out any excess water.
In a blender, combine the spinach, eggs, salt, and milk. Blend until smooth. Alternatively, you can use a food processor or immersion blender.
Add the flour and pulse until just combined. Avoid overmixing.
The batter should be thin and smooth, with a consistency similar to heavy cream. It should easily coat the back of a spoon and pour smoothly from a ladle without clumps. If the batter is too thick, add a little more milk, one tablespoon at a time, until it reaches the desired consistency. This will allow the crepes to spread thinly and cook evenly.
Heat a skillet over medium heat and lightly grease it with butter. Pour a small amount of batter into the pan and swirl it around to coat evenly. Cook each crepe for 1-2 minutes on each side, greasing the pan with butter before cooking each new crepe.
For one perfect crepe, use about ¼ cup of batter for an 8-10 inch skillet. Pour the batter into the center of the pan and quickly swirl to cover the entire surface before it begins to cook. This method creates light, delicate crepes.