Preheat oven to 220°C. To make the buns put the water, milk, butter, salt and 3/4 tsp of caster sugar into a saucepan, boil for 1 min stirring continuously. Take off heat, add flour, continue stirring until smooth. Return to heat, stir continuously for another minute to dry the paste a little then pour into a bowl. Quickly mix in eggs one at a time. Keep stirring until the paste is smooth then transfer into a piping bag with a 5mm round nozzle. On a tray lined with greased baking paper, pipe the choux paste into 3cm rounds. Bake for 5 mins before opening the oven door and leaving it slightly ajar and cook for a further 10-15 mins. Once cooked remove and allow to cool before pricking each choux bun with the tip of a knife so as to inject the filling.
Filling, whisk the thickened cream with the coffee until stiff. Gently fold in custard, transfer coffee mousse into a piping bag with a 5mm round nozzle. Pipe mousse into each choux bun and chill. Melt the chocolate in a bowl over a pot of boiling water. Dip each choux bun into the melted chocolate and serve.