Sift the flour into a bowl and stir in a pinch of salt. Put butter and 200ml cold water into a pan, bring to a rolling boil, then take off the heat. Tip in the flour and salt and stir to form a smooth paste. Now beat the mixture until it starts to come away from the sides of the pan, then tip onto a plate and leave to cool.
Heat oven to 200C/fan 180C/gas 6. Return paste to the pan, then gradually beat in the eggs, mixing well. Spoon walnut-sized balls onto a baking sheet, then bake for 20-25 mins until golden. Remove from oven, poke the underneath with a teaspoon, turn upside-down, then bake for 5 mins to dry out. Cool on a wire rack before filling.
For the filling, mix the custard powder and sugar with a splash of milk until you have a smooth paste. Heat the remaining milk, then stir into the paste. Pour the mixture into a pan and bring to the boil, stirring. Cook for 5 mins until you have a thick custard. Stir in coffee, then leave to cool. Softly whip the cream, sift in the icing sugar and mix, then fold into the
custard. Pipe the cream into each choux bun.
For the sauce, melt chocolate and butter with the coffee in a bowl in the microwave on Medium for 1-2 mins. Pour in the liqueur, mix well, then pour over the filled buns.