Preheat oven to 170°C / 340°F and line a muffin tin with paper liners or grease the muffin cups.
Melt butter in a microwave in 30 second increments, so it doens not explode all over the microwave. Take out carfully and let it cool a bit if it's very hot.
In a large bowl, whip the eggs and coconut sugar or sweetener of your choise together for about 3 minutes until pale and foamy.
Add yogurt, melted butter, mashed bananas, and vanilla extract to the wet ingredients.
Stir until everything is well combined.
Add almond flour, baking powder, baking soda, and salt to the bowl.
Mix until a thick muffin batter forms.
Fold in the chopped walnuts using a wooden spoon or spatula.
Divide the batter evenly between muffin cups, filling them nearly to the top. Don't worry they won't expand like regular flour muffins.
Add banana slices and walnut halves on top if desired.
Bake for 20–22 minutes until the muffins are golden brown and set.
To check if ready, poke a wooden skewer or toothpick all the way though and if it comes out clean it's ready. If still a bit wet then bake for another 5 minutes.
If the top is browining too fast, cover it loosely with alluminum foil and keep baking for 5 minutes until cooked though.
Allow them to sit in the muffin tray for about 15 minutes to cool before removing them, this will prevent them from ripping and breaking when taking out.