Place the water, butter, and salt in a small saucepan. Add all of the flour in one go. Work out any lumps, until a thick paste forms.
Transfer the dough to a pastry bag fitted with a small round pastry tip. Pipe 1 inch rounds of the pastry well-spaced apart.
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Transfer the dough to a mixer. Beat the eggs into the dough one at a time.
Gently brush with a little egg yolk and transfer to the oven. Bake the choux for 20-25 minutes or until golden.
In a saucepan, whisk to combine the sugar, cornstarch, lemon zest, and salt. Whisk in the water and lemon juice.
Whisk in two ladlesful of the hot lemon mixture into the egg yolks. Pour the yolk mixture into the lemon mixture and whisk until the filling thickens.
Transfer the lemon filling to a piping bag fitted with the #12 round tip.
Dunk the top of each puff into the meringue and toast the tops with a kitchen torch until golden.