Preheat the oven to 220 degrees Celsius (430 Fahrenheit).
To make the muffins, sift the flour in a large mixing bowl, add the ground cinnamon, baking powder and sugar, and mix well.
In a jug, crack the eggs in, and use a fork to beat them well.
Add the melted butter and buttermilk and mix again.
Peel, core and chop the apples in small cubes, then add to the the dry ingredients together with the wet ingredients.
Use a spatula to mix everything into a thick batter.
To make the topping, mix the flour, cinnamon and sugar, then add the melted butter and mix again.
Chop the walnuts roughly.
Grease and flour a 12-hole muffin tin, divide the batter evenly, then top with the buttery crumbs and chopped walnuts.
Bake for 5 minutes, then decrease the temperature to 180 degrees Celsius (350 Fahrenheit) for 20 minutes.