a piping bag with a French star tip nozzle, optional; a piping bag with a decorative tip, optional
For the eclairs: Put the water, milk, butter, superfine sugar and salt into a saucepan over medium heat and bring the mixture to a gentle boil; this should take about a minute. As soon as it reaches a boil, add the flour and cook, stirring constantly with a wooden spoon, until it forms a glossy ball of dough, about 2 minutes.
Put the ball of dough into a large bowl and allow it to cool for 2 minutes. Next, slowly add one-quarter of the beaten egg mixture and mix together with a wooden spoon until the mixture is homogeneous. Keep adding the egg a little at a time until the dough reaches the dropping stage--when you lift the wooden spoon out of the bowl, the mixture should fall off the spoon in 3 seconds. You may not need all of the egg, so be careful to add it slowly as to not make the mixture too runny.
Transfer the dough to a piping bag fitted with a French star tip nozzle (see Cook's Note). Line a baking sheet with a silicone mat or parchment paper and pipe ten 5-inch lines of dough on it. Put the baking sheet in the freezer for 20 minutes.
Preheat the oven to 205 degrees C/400 degrees F.
Just before you add the eclairs, throw 2 tablespoons of water in the bottom of the oven to create some steam. Immediately place the eclairs in the oven and lower the temperature to 160 degrees C/320 degrees F. Bake until the eclairs are golden brown, 30 to 35 minutes. Allow to cool.
For the filling: Add the cream, superfine sugar and vanilla to a bowl and whisk until it reaches very soft peaks. Place the mixture into a piping bag fitted with a French star tip nozzle or other decorative tip.
Assembly: Slice the cooled eclair shells in half lengthwise to make top and bottom shells. Dust the top shells lightly with powdered sugar. On the bottom shells, lay down some sliced strawberries, then pipe the whipped cream in a swirling motion on top. Place the top shells on the cream, then pipe more whipped cream on the tops in small dollops and decorate with more fresh strawberries.
If you do not have a French star tip nozzle, just pipe normal lines and gently score the dough with the end of a fork to create small ridges.