Make the choux pastry

• Heat the water and butter in a saucepan until the butter melts and the mixture boils
• Add the flour and stir vigorously until a smooth dough forms
• Remove from the heat and let cool slightly

Add the eggs

• Beat in the eggs one at a time until the dough is smooth and glossy

Pipe and bake the puffs

• Pipe the dough into rounds on a baking tray
• Bake in a preheated oven at 200 °C for 10 minutes
• Lower the temperature to 170 °C and bake for an additional 30 minutes

Make the custard

• Whisk the egg yolks, sugar and cornstarch together
• Heat the milk and pour it over the yolk mixture while whisking
• Return to the heat and cook, stirring, until thickened

Whip the cream

• Whip the heavy cream with the sugar until stiff peaks form
• Fold in the melted dark chocolate if using

Assemble

• Fill the cooled puffs with the custard and the whipped cream