To make the choux, tip the butter into a small saucepan with 100ml water, the caster sugar and a pinch of salt. Bring to the boil, then, still over the heat, add the flour, beating vigorously until the mixture clumps and comes together into a ball that pulls away from the sides of the pan. Remove from the heat and leave to cool in the pan until just warm. Gradually beat in all but about 1 tsp of the egg using an electric whisk. Tip the pastry into a piping bag.

Heat the oven 200C/180C fan/gas 6. Trace a 10cm circle on a piece of baking parchment using a small bowl or lid, then flip it over onto a baking tray. Pipe a thick ring of the pastry around the outside of the circle. Brush the pastry with the remaining egg, then scatter over the almonds. Bake for 15 mins, then reduce the oven to 160C/140C fan/gas 3 and bake for 20 mins more. Reduce the oven again to 110C/ 90C fan/gas 1/4 and bake for a final 10 mins. Turn off the oven and leave the ring to cool inside for 30 mins without opening the oven door.

Meanwhile, make the filling. Gently warm the custard in a small pan over a low heat, then stir in the dark chocolate, continuing to stir until the chocolate has fully melted and the mixture is thick. Scrape into a bowl and leave to cool. Whisk the chantilly cream ingredients together in a separate bowl until very thick and chill until needed.

Carefully halve the choux ring through the middle using a serrated knife, and gently lift off the top. Spoon the chocolate filling into a piping bag and pipe large blobs over the bottom ring, leaving a gap between each. Put the chantilly mixture in a second piping bag and pipe blobs in the gaps between the chocolate filling. Carefully sandwich with the top of the ring, then dust lightly with the icing sugar and serve.