MAKING THE PASTRY
Preheat the oven to 320°F (160°C)
Combine the butter, water and milk in a saucepan. Add salt and sugar.
Place over medium heat and bring just to a boil, so that the butter melts completely. Remove the pot from the heat as soon as the mixture begins to bubble.
Reduce the heat to low heat, add flour, all at once. Place the pot on the stove again. Allow the mixture to cook for 2 minutes while whisking continuously at all times to sticking or burning and to achieve a nice, thick texture. SEE NOTE 3
Transfer the mixture to a mixing bowl and beat for 1 minute to cool the mixture slightly before adding the eggs. SEE NOTE 4
Add one egg at a time, adding another egg only when the previous has been completely incorporated. Don't forget to mix at all times, using an electric mixer.
Spoon the dough into a piping bag fitted with a round tip nozzle.
Pipe the pastry onto a baking sheet lined with parchment/baking paper. Each pastry should be piped around 1½ inch ( 4cm) in diameter. You might get peaks on the pastry while piping, just smooth it out by wetting your fingertips with water and smoothing out the peaks.
Bake for about 25-30 minutes. Don't be tempted to open the oven door for the first 20 minutes or the pastries will collapse.
Remove from the oven and transfer the baked pastries to a cooling rack. SEE NOTE 5
Cool completely before adding the whipped cream SEE NOTE 6
MAKING THE WHIPPED CREAM
Combine the cream, sugar and vanilla extract in a bowl. It is best to use chilled cream as it is easier to beat.
Beat the mixture on medium speed for about 2 minutes or until stiff peaks form in the cream. Do not overmix, or it will ruin and separate the cream.
Cut the tops off the cooled pastry and pipe or spread with the whipped cream. Remember that the pastry must be cooled completely before filling with cream. Fill the pastry just before serving. Filling it too early before serving can cause the pastry to lose its crisp texture.
Cover with the tops and dust with powdered sugar.