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The icing will need to chill in the fridge overnight, so make it the day before you want to serve the cakes. Sift the icing sugar and custard powder into a large bowl, add the vanilla extract and 4 tbsp of the cream, then whisk until smooth.
Tip all but 2 tsp of the remaining cream into a medium saucepan and heat gently until small bubbles start to appear around the edges of the pan – the cream should be hot, but not boiling. Gradually pour this into the bowl with the custard powder mix, whisking all the time. Pour the mixture back into the saucepan and return to the heat. Cook gently for 4-6 mins, stirring all the time, until the mixture has thickened and the custard powder has dissolved. Transfer to a bowl and cover the surface directly with cling film. Leave to cool, then chill overnight.
The next day, heat the oven to 180C/160C fan/gas 4. Line a 12-hole muffin tin with muffin cases. Toss the apples and custard powder together in a bowl and set aside. In a large bowl, beat the butter and sugar together, using an electric whisk, until light and fluffy. Slowly add the eggs and vanilla extract, whisking well after each addition. Fold in the flour, then stir through the custard powder- coated apples. Divide the mixture between the muffin cases and bake for 18-20 mins until lightly golden, and a skewer inserted into the centre comes out clean. Leave to cool on wire racks.
Blitz all the ingredients for the crumble together in a food processor until the mixture resembles fine crumbs. Tip half the mixture onto a baking tray lined with baking parchment and bake for 5-10 minutes until lightly golden.
Continue to pulse the remaining half of the crumble topping mixture until it comes together to form a dough, adding 1-2 tsp of the reserved double cream to bind, if needed. Form the dough into a ball, wrap in cling film and chill for 30 mins, or until firm.
Roll out the dough on a lightly floured surface to the thickness of a £1 coin. Using an apple-shaped and a leaf-shaped cutter, or a sharp knife, cut out 12 apple shapes and leaves. Lift the shapes with a palette knife and transfer to a baking tray lined with parchment. Bake for 8-10 mins until lightly golden. Leave to cool for 5 mins, then transfer to wire racks to cool completely.
Melt the chocolate in a small bowl over a pan of simmering water, or pop in the microwave for 5-10 second bursts. Dab each leaf biscuit with a little of the melted chocolate and fix it to an apple biscuit. Leave to set.
Whip the chilled custard cream icing to soft peaks, then transfer to a piping bag fitted with a large round nozzle. Pipe some icing onto each cupcake, then smooth the surface using a small metal spatula or the back of a spoon. Sprinkle over some crumble, then insert an apple biscuit into the top.