Line a 12 hole muffin tin with paper cases and preheat your oven to 190 degrees celsius (fan-forced).
Cube the butter and place into a microwave safe bowl. Cook in 30 second spurts until it has just melted.
Carefully pour the melted butter into a large bowl and add the brown sugar and cinnamon, stir to combine.
Place the milk into a jug, add the egg and vanilla extract. Use a fork or a whisk to combine.
Add the flour and baking powder to the bowl with the melted butter and sugar. Add the liquid ingredients and use a whisk to mix until almost combined - it's fine to still have a few small lumps in the mixture.
Add the chopped apple pieces and gently fold through with a spoon.
Divide the muffin mixture between the paper cases in the muffin tray. Sprinkle the raw sugar over the top of the muffins if you are using it.
Place the muffins into the oven and bake for 18 - 20 minutes or until cooked through when tested with a skewer. Carefully remove the muffin tray from the oven and allow the muffins to cool for 10 minutes before transferring to a wire rack to cool completely.