In a bowl, add flour, baking powder, baking soda, salt and cinnamon together. Stir to combine.
In a second bowl add yogurt, milk, eggs, sunflower oil and maple syrup. Whisk together with a whisk.
Peel the apple, remove the core and cut into small cubes.
Pour the yogurt mixture into the bowl of flour and use a spatula to make it into a batter. It does not matter if there are still some lumps of flour in it.
Add the diced apples and fold them evenly through the batter.
Pour the yogurt mixture into the bowl of flour and use a spatula to combine into a batter. It does not matter if there are still some lumps of flour in it.
Optional: sprinkle some coarse cinnamon sugar over the muffins.
Bake the yogurt muffins for 20-22 minutes at 200 °C/400 ℉ (conventional) or until golden brown.
How to store
Outside the refrigerator in an airtight container, the muffins will keep for 3-4 days. In the freezer, they will keep for up to 3 months.