Preheat the oven to 375 degrees F and grease a muffin tin well with nonstick spray.
Place all ingredients into a blender except the flour and chocolate chips, if using.
Blend until very smooth, stopping to scrape down the sides of the bowls as needed. You want the greens to be very finely blended in so the batter is green, as if you were making a smoothie.
Pulse in the flour just to combine, or stir it in gently.
Pour batter into the prepared muffin tin, filling each cup about 3/4 cup full. Sprinkle with chocolate chips if using.
Bake for 18-20 minutes or until firm to the touch and lightly browned around the edges. (You can also check to see if a cake tester inserted into the center comes out cleanly.)
Remove from the oven and use a paring knife, if needed, to transfer to a wire rack to cool.
Serve slightly warm, at room temp, or chilled.
You can store the muffins in an airtight container in the fridge for 3-5 days and eat cold or slightly warmed. Or store in a zip top bag in the freezer for up to 3 months. Thaw overnight in the fridge or at room temperature.