Preheat oven to 180C/350F and grease a 12 hole muffin tin with butter.
If you are going to be using the topping, prepare this first. Add sugar, almond meal and melted butter to a bowl and mix until well combined. Set aside.
Mix the sourdough starter, 2 eggs, vanilla extract and oil in a bowl. Whisk together until well combined. It's ok if the oil sits on the top, it will combine once it is poured into the dry ingredients.
In a separate bowl mix together all purpose flour, salt, baking powder, cinnamon and sugar. Once combined, add the diced apple and mix until all of the apple is coated in the flour mixture.
Now pour the liquid ingredient mixture into the dry ingredients. Gently fold together until just mixed - don't over mix or the muffins will be tough.
Spoon mixture into greased muffin tin. This recipe will fill a 12 hole muffin (each hole around ¾ full).
If you are adding the almond crunch topping, spoon a small amount into the centre of each muffin (it will spread out as the muffins cook).
Bake at 180C/350F for approximately 20 minutes or until muffins are golden brown. If using the almond sugar topping, just watch them as they can burn easily.