Make the Crumble Topping: To a medium sized bowl, add all the ingredients for the crumble topping and using your fingers, rub in the butter until the mixture resembles breadcrumbs. You can also complete this step in a food processor. Set the crumble mix aside.
Make the Muffins: Preheat the oven to gas mark 4/350oF/180oC and line a 12-count muffin tin with 12 muffin cases.
In a bowl, whisk together the flour, salt and cinnamon and set aside. In a mixing bowl, combine the butter, sugar and vegetable oil until smooth.
Mix in the eggs, vanilla extract, soured cream and milk. Gently fold in the chopped apples then mix in the dry ingredients set aside earlier. The batter will be quite thick once all the ingredients are combined.
Evenly distribute the batter between the 12 muffins cases. The cases will be around ¾ full. Using a spoon sprinkle the crumble topping onto each muffin and gently press the crumble into the batter to ensure it bakes into it.
Bake in the middle of the oven for 25 mins or until a cake tester comes out clean then remove from the oven and allow to cool in the pans for 15 minutes. After 15 minutes, transfer the muffins to a cooling rack to completely cool.