Whisk the butter in a bowl and add the brown sugar all purpose flour, yogurt, baking soda, diced apple, cinnamon powder, and egg. Mix well to form a smooth batter. Pour the batter into silicon moulds. Place them on a baking tray bake in a pre heated oven at 200 degrees Celsius for 15 20 minutes. Serve the muffins with a cold, refreshing drink.