To make the filling, put the custard powder into a bowl, add the sugar and 2 - 3 tablespoons of milk and blend to a smooth paste. Heat the remaining milk in a saucepan until almost boiling, then pour it into the custard paste and stir it in. Return the custard to the saucepan and gently bring to the boil. Stir well until the custard thickens.

Cook for 1-2 minutes, until no trace of the custard powder remains, then beat in the vanilla essence. Pour into a clean bowl and cover with plastic film. Allow to cool, then refrigerate for 2 - 3 hours.

Meanwhile, to make the pastry, put the margarine or butter into a saucepan with 7 1/2 fl oz (215 ml) of cold water. Heat gently until the margarine melts, then bring to the boil. Tip the flour into the saucepan and beat quickly, until the mixture forms a stiff paste. Remove from the heat. Whisk 2 eggs and 1 egg white together, then gradually whisk into the paste.

4.Spoon the paste into a piping bag fitted with a plain 1/2 in (15 mm) nozzle. Line 2 baking sheets with nonstick baking paper and pipe 3 in (80 mm) lengths of paste, spaced well apart, onto the paper.

Bake in the Oven for 20 - 25 minutes, until well risen, golden-brown and quite firm. Remove from the Oven and make a small hole in the end of each eclair with a sharp knife, to allow steam to escape. Return to the Oven for 5 minutes to dry. Remove from the Oven and leave to cool on a wire rack.

Whisk the chilled custard mixture until very smooth, then whisk in the yoghurt. Whisk the cream until stiff, then fold it into the custard. Place this mixture into a clean piping bag and pipe the pastry cream into the hole in each eclair. Shake to ensure the cream is evenly distributed.

Place the icing sugar in a large bowl and dissolve the coffee granules in 1 teaspoon of boiling water. Add the coffee to the icing sugar and add just enough boiling water to make a thick, smooth icing. Dip the top of each eclair in the icing, lift out and drain off any excess. Place on a rack until the icing sets.

If using the chocolate, melt it in a small pan and pour into a small paper piping bag. Cut a small hole in the end of the bag and use to pipe chocolate decoratively. Keep refrigerated until ready to serve.