Preheat to 200°C (400°F).
In a medium pot, bring the water and butter to a gentle boil. Stir until the butter is fully melted.
Remove the pot from the heat and quickly add the flour and cornflour. Stir vigorously until the mixture forms a smooth ball that pulls away from the sides of the pan.
Return the pot to heat and stir for 1 minutes to dry out the dough slightly.
Transfer the dough to a mixing bowl.
Using a hand or stand mixer beat in the eggs one at a time. Use as many eggs as necessary (4-6) until the dough is glossy and it drops off a spoon in long peaks. Add the baking powder to the cooled dough and mix.
Spoon the dough into a piping bag fitted with a round tip. Pipe small rounds for cream puffs or longer strips for éclairs onto a lined baking tray. Use a template as a guide for even pastries. (Remember to remove the template after piping)
Place in the oven and bake for 20-25 minutes until puffed and golden brown. Do not open the oven during the first 15 minutes or the pastry may collapse.
Remove from the oven and immediately cut them open. Allow to cool completely before filling with cream or other desired fillings.