Preheat oven to 180 C, 10 mins before baking.
Mix together the streusel ingredients and keep aside.
Combine flour, sugar, baking powder, salt, and cinnamon. Whisk together the egg, milk, and melted butter. Pour the wet ingredients over the dry, and stir just until combined.
Fold in the apples. Pour into lined muffin cups and sprinkle the streusel topping over each muffin.
Bake in the preheated oven for 20-25 mins.