Preheat the oven to 220C/200C Fan/Gas 7. Line a baking sheet with baking paper and draw a 10cm/4in circle in the middle. Turn the paper over so the pencil marks face down.
For the choux, sift the flour straight onto a piece of baking paper. Place the butter and 5 tbsp water in a small saucepan and place over a low heat until the butter is melted. Bring to the boil and remove from the heat as soon as it boils. Tip all the flour in and beat vigorously with a wooden spoon until the dough comes away from the sides of the pan. Allow to cool slightly, then gradually beat in the egg, beating well between each addition until the dough is smooth and glossy. Spoon the dough into a piping bag and snip off the tip to make a generous hole, approximately 2cm/¾in in diameter (alternatively use a large plain wide nozzle).
Pipe a thick ring onto the circle on the baking paper, making sure you pipe the ring in one motion. Brush with the egg yolk and sprinkle with flaked almonds. Stick down the corners of the baking paper using any leftover dough (this will prevent the paper curling up in the oven).
Bake for 20 minutes, or until well risen and golden brown. Remove from the oven and allow to cool slightly. Slice the ring in half horizontally. It should be soft in the centre and slightly crisp on the outside. Dry it out in the residual heat of the oven if necessary.
For the chocolate hazelnut filling, put the chocolate hazelnut spread in a large bowl and add the double cream. Whip together until soft peaks form when the whisk is removed. Spoon into a piping bag fitted with a large star nozzle (alternatively snip off the corner).
For the Chantilly cream filling, place the double cream in a large bowl and add the sifted icing sugar and vanilla seeds and whip together until you reach soft peaks. Spoon into a piping bag fitted with a large star nozzle (alternatively snip off the corner).
For the chocolate sauce, melt the chocolate, 140ml/4½fl oz water and sugar together in a medium saucepan until smooth. Turn off the heat and transfer to a small jug.
Pipe generous amounts of the chocolate filling and Chantilly cream onto the pastry base. Top with the pastry lid and dust with icing sugar. Serve the chocolate sauce in a jug alongside.