Heat the water and butter gently in a saucepan until melted. Bring to the boil then add the flour and beat until it forms a smooth ball that leaves the sides of the pan almost clean. Leave to cool for 10 minutes.
Gradually mix in the eggs and vanilla until thick and smooth. Spoon the choux mixture into a large nylon piping bag fitted with a 1 cm (½ inch) plain piping tube and pipe 7.5 cm (3 inch) lines of mixture on to a large lightly greased baking sheet.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes until well risen. Make a slit in the side of each éclair for the steam to escape then return to the turned-off oven for 5 minutes. Leave to cool.
Whip the cream to soft swirls then gradually whisk in the icing sugar and liqueur. Slit each éclair lengthways and spoon or pipe in the cream.
Make the chocolate topping. Heat the butter, chocolate and icing sugar together gently until just melted. Stir in the milk then spoon over the top of the éclairs. Serve the same day.