To make the choux pastry, pour 85ml of water and the milk into a medium pan with the butter, sugar and salt. Cook over a low-medium heat until the sugar and salt have dissolved, then turn up the heat to medium-high and bring to a rolling boil. Remove from the heat, add the flour and quickly stir to make a dough. Return to the heat and cook for 2-3 minutes, stirring constantly, until a film forms on the bottom of the pan. Tip the pastry into a bowl and beat for a couple of minutes until cooled. Add the eggs, a little at a time, beating in each addition until fully combined before adding more. Add just enough of the beaten eggs to make a slightly glossy dough that, when lifted from the bowl, falls from the spatula in a V-shape (you may not need all the eggs). If it tears when lifted, it needs a little more egg; if it’s too loose and flows easily, you’ve added too much.