Butter a 12-piece muffin tin and set aside
Defrost the berries, then drain well. Peel, quarter, core and grate the apple. Put the berries and apple in a large bowl. Add maple syrup, yoghurt and muesli and mix everything carefully
Mix the cane sugar with the oil. Add eggs and milk and mix everything well and thoroughly with a whisk. Mix the flour, baking powder, baking soda and salt in a second bowl, then add to the liquid ingredients and stir with a wooden spoon or rubber spatula only briefly until all the ingredients are just mixed. Add the muesli mixture and fold in only briefly
Spread the dough evenly on the muffin tin. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for about 20 minutes. Make a cooking test with a wooden skewer. Serve warm with butter