1 tbsp lemon juice / cider / distilled white vinegar
Preheat the oven to 190 degrees Celcius. Line muffin moulds with paper liner / spray with non stick cooking grease / brush with melted butter.
Sift the dry ingredients; flour, baking powder and soda into a large bowl. Stir in salt and sugar and make a well in the centre.
In a separate bowl, combine all the wet ingredients; eggs, buttermilk, oil and vanilla extract and mix well.
Pour the combined wet ingredients into the well and fold until the batter is just mixed; the batter should look quite lumpy.
(Avoid over-mixing the batter as this will cause the muffins to be tough and chewy in term of texture).
Spoon the batter 1/2 full into the prepared moulds and pipe the apple filling in the centre of the batter and fill up more batter till 3/4 full. Sprinkle crumble topping on the entire surface of the batter.
Crumble Topping: Combine all the dry ingredients in a bowl. Rub in butter until mix resembles bread crumbs. Sprinkle as topping.
Bake for 20 - 25 minutes in preheated oven or until golden brown.
Cool on a wire rack before unmoulding the muffins and dust with confectioners' sugar if necessary.
Buttermilk: Combine both and allow to stand for 10 minutes before use.