Heat the oven to 200C/180C fan/gas 6. Line a large baking sheet with baking paper and mark 8 circles about 8-9cm across using a glass or mug. Turn the paper over. Sift the flour onto a sheet of baking paper. Put the butter and 150ml water into a pan, then bring slowly to the boil until the butter has melted. When the water is bubbling, remove quickly from the heat, tip in the flour all at once and beat quickly with a wooden spoon until it forms a dough that leaves the side of the pan clean. Remove from the heat and cool for 3 mins.
Lightly beat the eggs with a fork. Add the egg to the dough gradually, beating well with each addition until the dough is smooth and glossy. Put this dough into a piping bag fitted with a large plain nozzle (about 2cm across) or use a large food bag with the corner snipped off. Pipe 8 thick rings onto the baking sheet using the marked circles as a guide.
Bake the buns for 20-25 mins until crisp and golden, then split them carefully in half through the centre using a sharp knife in a sawing action; return to the oven for 5 mins. Transfer to a cooling rack, taking care to keep the tops and bottoms together, and leave to cool. Whip the cream to soft peaks and stir in the vanilla. Melt 100g of the chocolate for 1-2 mins in a microwave. Leave to cool slightly, then fold into the cream.
Stir half the crystallised fruit and the almonds through the cream. Fill the choux rings with the mixture using two teaspoons. Melt the remaining chocolate and drizzle over the choux rings. Scatter with crystallised fruit and leave to set. Dust with icing sugar before serving.