Preheat the oven to 200 degrees Celsius (392 degrees Fahrenheit) fan. Bring water and butter to a boil. When butter and water are boiling, add the flour and stir quickly. When the dough leaves the sides of the pan, remove the pan from the heat. Let the dough cool slightly (if the dough is too hot when the eggs are beaten in, the eggs and dough will not combine). When the dough has cooled a bit, beat in the eggs one at a time (you can use a hand mixer). When all the eggs are beaten in and the dough has come together, put it in a piping bag with a star nozzle. Pipe the dough onto a baking sheet lined with baking paper. Bake the choux pastry in the middle of a hot oven for 15-30 minutes. Do not open the oven for the first 15 minutes, or the choux pastries will collapse. Cut them open and fill with whipped cream and pastry cream. They are now ready to serve.