Stir water and salt together in a pot

Add chopped butter pieces and melt over medium heat

Once the mixture starts to boil, add the flour and stir immediately

Keep stirring until the dough becomes smooth and thick, about 1 to 2 minutes

Dry the dough for another 3 to 5 minutes to evaporate excess moisture

Transfer the dough to a new bowl and let it cool for about 5 minutes

Add eggs one at a time, thoroughly mixing after each addition

Add the last egg gradually by beating it separately first, then mixing in small batches

Transfer the dough into a pastry bag or plastic bag with a corner snipped off

Pipe out small rounds roughly the size of a ping-pong ball onto a baking tray, spacing them out

Dampen your fingertip and press down the pastry peaks for a smoother shape

Bake at 190°C (375°F) for about 10 minutes

Reduce temperature to 170°C (335°F) and continue baking for another 30 minutes

Do not open the oven for the first 20 minutes to avoid collapse

Combine flour, sugar, and salt in a bowl for the custard

Prepare three egg yolks at room temperature

Heat milk in a pan with sugar sprinkled at the bottom

Heat milk until small bubbles appear around the edges, but do not fully boil

Slowly add the dry ingredients to the milk while whisking fast

Keep cooking and stirring until the mixture thickens, about 1 to 2 minutes

Slowly pour about half a cup of the hot mixture into the eggs in a thin stream

Gently pour the egg mixture back into the pot a little at a time, stirring as you go

Keep stirring until the cream thickens up nicely

Cook on low heat for about 5 to 7 minutes until silky and smooth

Stir in vanilla extract if desired

Pour the custard through a sieve into a chilled container

Cover with plastic wrap pressed directly onto the surface

Refrigerate for at least 30 minutes

Whip heavy cream until it reaches stiff peaks

Gently fold the chilled custard into the whipped cream until smooth and well combined

Transfer the cream filling into a pastry bag

Fill the cooled cream puffs with the custard cream mixture