Prep the oven and pan: Preheat to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or grease well. For mini muffins, grease a 24-cup mini muffin tin.
Blend the greens and fruit: In a blender, add bananas, spinach, milk, oil, egg, vanilla, and maple syrup. Blend until completely smooth and bright green.
No visible spinach pieces should remain.
Mix dry ingredients: In a large bowl, whisk flour, baking powder, baking soda, cinnamon, and salt.
Combine: Pour the green mixture into the dry ingredients. Stir gently with a spatula until just combined. A few small lumps are fine.
Do not overmix or the muffins can turn dense.
Fold in extras: If using add-ins, fold them in now. Keep amounts modest so the muffins stay soft and toddler-friendly.
Fill the cups: Divide the batter evenly. For regular muffins, fill about 3/4 full.
For mini muffins, fill nearly to the top.
Bake: Bake regular muffins for 16–20 minutes or mini muffins for 10–12 minutes. They’re done when a toothpick in the center comes out clean or with a few moist crumbs.
Cool: Let muffins rest in the pan for 5 minutes, then move to a wire rack to cool completely. This helps set the texture and prevents sogginess.
Serve: Offer plain or with a smear of cream cheese, yogurt, or nut/seed butter for extra protein and healthy fats.