There's a recipe below, but all you REALLY need to remember when making muffins:
VOLUMES: The ratio of Flour:Liquid:Egg:Oil is 2:2:1:1 (by weight)
MIX: Stir liquids GENTLY into the dry ingredients
TIME: Muffins are ready when you can put a toothpick into the muffins, and it comes out without dough on.
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Preheat your oven to 200°C (400°F).
Finely chop your chocolate.
Mix your dry ingredients: Baking powder, cinnamon, flour and sugar
Mix your wet ingredients in a bowl: butter, eggs, milk, nutella and sugar
Gently blend the wet ingredients into the dry ingredients.
Fold in the chocolate.
Distribute a thin layer of butter or vegetable oil in each tin in your muffin tray, to prevent the sponge from sticking to the sides during cooking. Then distribute the dough in the tins.
Bake your muffins for about 30 minutes at 200 C. They are done when a toothpick pricked in the middle comes out clean.
Sprinkle the ganache and icing on top before eating.