Add the milk, banana flesh, your eggs, your melted butter and vanilla extract to a blender, and then blend until smooth and well combined.
Put a muffin case in each dip of a 12 hole muffin tin. Then spray each muffin case with cooking spray.
Transfer the batter to each muffin case evenly, then bake in the oven for 30 minutes.
Put a skewer in the middle of a muffin, and take it out. If the skewer is clean, the muffin is cooked, if there appears to be some batter on the skewer, continue cooking the muffins for 3-5 more minutes.
From there, transfer the muffins on a cooling rack if you have one, then remove the muffins from the cases once they have cooled enough.
Blend the banana flesh and milk and a blender until you have a smooth mixture.
Use an electric mixer to beat the butter for a few minutes in a large bowl, until fluffy and pale in colour.
Finally, add the blended banana and milk, then whisk through one last time.