Bring the milk, water, salt and sugar to a boil in a small saucepan. Don't leave it to boil as the water will evaporate.
Remove the mixture from the heat and add all of the flour in one go. Stir well with a wooden spoon and place the pan back onto the heat.
Continute to stir while the mixture cooks and the dough drys, coming away from the sides of the pan. It will form a soft dough.
Remove the pan from the heat and add the eggs, one at a time, beating well with a spatula between each addition.
The dough will become firm, but will still be soft enough to drop off the spoon. If it's too thick add a little hot milk to loosen it up.
Transfer the pastry mixture to a piping bag fitted with a 1/2 inch plain nozzle.
Pipe 4 inch long fingers onto a lightly greased baking tray, leaving room for them to spread.
Sprinkle the tray with a few drops of water and bake for 15 minutes, leaving the door closed. Then, open the door slightly and bake for a further 10 minutes or so until the pastry is crisp and golden-brown.
Remove the pastry from the oven and, using a small toothpick, carefully poke a small hole in the side of each pastry to let the steam escape. Bake for a further 5 minutes. Remove from the tray and leave to cool completely on a wire rack.
For the Crème Pâtissière
Split the vanilla pod down the side and place a saucepan along with the milk and 1/3 of the sugar. Bring to a boil.
Whisk the remaining sugar with the egg yolks in a bowl. Add the cornstarch and whisk well until combined.
Remove the vanilla pod from the milk, reserving the seeds.
Pour a little of the hot milk into the egg yolk mixture and whisk until combined.
Add the remaining milk and return the pan to the heat, whisking continually until the custard mixture thickens.
Transfer the thickened mixture to a bowl and add the butter. Stir well until combined. Cover and chill in the refrigerator until needed.
For the Fondant Frosting
Let the fondant melt in a bain-marie. Add dark chocolate, as desired.
For the Chocolate Frosting
Break the chocolate and place in a heat proof bowl over a pan of barely simmering water along with the butter and water. Leave to melt and then stir in the icing sugar.
Slice the eclairs lengthwise and pipe them full with crème pâtissière. Top with the chocolate frosting of your choice.
To make Swans
Pipe shorter lines of choux pastry as well as large number two shapes for the swan neck.
To assemble, cut the pastry top off and halve to form two wings. Arrange the neck with the base of the number two hidden down the length of the body. Fill with piped Diplomat Cream, equal portions of plain whipped cream mixed with cooled crème pâtissière. Arrange the wings on top of the cream and dust lightly with icing sugar.