Preheat oven to 200 C / Gas 6. Grease or line a 12-hole muffin tin. Mix together 120g oatmeal, flour, baking powder, bicarbonate of soda and 1/8 teaspoon cinnamon; set aside.
Beat 110g dark brown soft sugar, egg white, apple sauce and milk together in a mixing bowl until smooth. Stir in the flour mixture until evenly moistened; it's ok if small, dry lumps remain. Pour the batter into prepared muffin tin.
Stir 4 tablespoons oatmeal, 4 tablespoons dark brown soft sugar, 1 tablespoon cinnamon and the melted butter in a small bowl; sprinkle evenly over the muffins.
Bake in preheated oven until golden and the tops spring back when lightly pressed, about 20 minutes. Cool at least 10 minutes before serving.
If your local supermarket doesn't stock oatmeal, you can make your own by pulsing some porridge oats in a food processor until ground.