Sift the icing sugar and custard powder into a bowl and combine with vanilla extract and double cream. Mix until smooth and creamy for a luscious icing.

Gently warm the remaining double cream in a saucepan, then mix it with the custard to thicken. Let it chill in the fridge to set perfectly.

In a separate bowl, combine flour, rolled oats, brown sugar, and ground cinnamon. Add in the chilled butter and mix with your fingers until crumbly, then bake for 15 minutes until golden brown.

Melt the butter in a skillet over medium heat and add the chopped apples. Sprinkle with sugar and cinnamon; gently simmer until the apples are tender and fragrant.

In a mixing bowl, cream together the unsalted butter and caster sugar until fluffy. Gradually add the dry ingredients, then stir in the eggs, milk, and vanilla extract, folding until just combined.

Fill the cupcake cases halfway, add a spoonful of the apple filling, and top with more batter. Bake for 18-20 minutes until golden and a toothpick comes out clean. Let the cupcakes cool in the tin for a few minutes, then transfer to a wire rack to cool completely.

Once cooled, pipe the chilled custard icing onto each cupcake generously, and finish off with a sprinkle of your delicious crumble topping.