Combine water and milk in a saucepan, add sugar, salt, and butter. Stir continuously until boiling, then remove from heat and add sifted flour, stirring for 2 minutes.
Let the dough cool for about 10 minutes, then add eggs one at a time, mixing well until smooth. Transfer to a piping bag and pipe onto a baking tray lined with parchment paper.
Bake at 180 degrees for 10 minutes in a preheated fan oven, then reduce temperature to 160 degrees for another 15 minutes.
While the eclairs cool, prepare the cream by heating 350ml milk in a saucepan. In a bowl, mix 150ml milk, eggs, vanilla, and flour.
Once the milk bubbles, reduce heat and add the mixture, stirring continuously until thickened. Finally, add butter and let cool.
Make a small hole in the bottom of each éclair and fill with cream using a piping bag.