Put the water and oil in a pot to boil. Then add all the flour at once and start stirring with a wooden spoon until homogenized and until it starts coming off the walls of the container. Set aside and leave to cool. Then add the eggs 1 by 1 and the egg white, beating using a mixer with dough beaters.
Heat the oven to 392°F (200 °C). Put baking paper in a tray, fill a pasty bag with the dough and spray out eclairs. You can use a spoon to form them if you don't have a pastry bag. Bake in the preheated oven 30-35 min.
Do not open the oven during that time, otherwise they'll deflate! During baking the eclairs will double in volume, so don't put them too close to one another. Take the ready eclairs out immediately without waiting for them to cool and spray the next batch onto baking paper. Repeat until out of dough.
For the cream, mix all of the ingredients together without the butter in a pot and while stirring nonstop put it on the stove. Stir energetically to avoid burning. Once boiling, remove from the stove and add the butter. Stir until it melts.
Wait about 30 min. for the cream to cool (cover with plastic wrap well so it doesn't catch a crust) and fill the eclairs with it (either with a pastry bag or cut them and fill with a spoon).