Grease a large baking sheet.
Heat the butter and measurement water gently in a saucepan until melted. Bring to the boil, then add the flour all at once and beat the mixture until it forms a smooth ball that leaves the sides of the pan almost clean. Leave to cool for 2 minutes.
Beat the eggs and vanilla extract into the paste gradually until thick and smooth. Spoon the choux pastry into a large piping bag fitted with a 1 cm (½ inch) plain nozzle, and pipe 7 cm (3 inch) lines of pastry onto the 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 for the filling 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 by heating the butter, chocolate and icing sugar together gently until just melted. Stir in the milk, then spoon over the top of the éclairs. These are best eaten on the day they are made.