For the chia seed banana muffins, mash or puree the banana well with a fork and mix with butter, sugar, vanilla sugar, chia seeds and oat drink until smooth.
Set aside and let the chia seeds swell a bit in the mixture. After about 10 minutes, combine everything with the remaining ingredients, mix into a batter and pour into muffin tins.
Bake the muffins for about 25 minutes at 160 degrees. After baking, sprinkle with oat or spelt flakes as desired.