beat the butter and sugar together until well combined, then beat in the flour and bring together to form a soft dough.
Place the dough between two sheets of parchment paper and roll out until 3mm (⅛ in). Slide onto a tray and freeze the dough.
Make the choux buns
Place the butter in a saucepan with the water. Heat gently until the butter melts then bring to a rolling boil.
Take off the heat and immediately add the flour in one go then beat well with a wooden spoon until the mixture leaves the sides of the pan and forms a ball around the spoon. Allow to cool.
Preheat the oven to 200ºC/400ºF/gas 6. When the paste has cooled add the eggs a little at a time and using an electric whisk, beat well after each addition.
Remove the craquelin from the freezer and let it stand while you pipe eight mounds of choux pastry about 5-6cm (2½in) onto a baking sheet.
Cut out eight 5cm (2in) discs of the craquelin dough using a biscuit cutter and place on top of each of the choux buns.
Bake for 20–25 minutes until well risen and golden and the top has cracked. Remove from the oven and poke a small hole into the side of each to allow the steam to escape. Return to the oven and cook for a further 5 minutes then transfer to a wire rack to cool completely.
Make the filling
Melt the butter in a frying pan and gently sauté the apples until begining to soften. Stir in the muscovado sugar and brandy and simmer for a few minutes until the apples are tender and the sauce thickens slightly.
Allow to cool completely.
Whip the cream with the vanilla extract until standing in soft peaks.
Cut the buns in half horizontally and divide the apple mixture between the buns. Pipe or spoon the whipped cream on top and replace the lids. Serve.
If making ahead of time – freeze or store in an airtight container. Refresh in a hot oven for 5 minutes then allow to cool again before using.