Preheat your oven to 375°F / 190°C.
Place the salt, water, sugar (if using), and butter in a saucepan. Heat over medium heat, stirring occasionally.
When the water starts to boil (the butter should be melted), remove the saucepan from the heat. Add the flour all at once. Vigorously mix the flour in with a wooden spoon or spatula until it absorbs all the water.
Return the pan to medium heat. Cook the dough for 2 to 5 minutes, mixing and moving it until the dough pulls away from the side of the pan. If you gather the dough on one side, a spoon stuck in the dough should stand upright.
Transfer the dough to a bowl and let it cool slightly for a few minutes. Whisk all the eggs together in a jug.
Add the vanilla extract (if using). Add the whisked eggs in 4 to 5 additions, mixing the eggs into the dough well between each addition using a handheld mixer or whisk. Only add enough egg to achieve a glossy, pipeable dough consistency; you may not use all the eggs.
Prepare a baking tray by lining it with parchment paper. Pipe circular mounds of the choux pastry dough, about 1.5 inches at the base and 1 inch high, using a piping bag fitted with a large round tip.
For craquelin tops, place a circular disc of the prepared craquelin cookie dough on top of each piped mound, centered, without pressing it down.
Bake for 30 to 40 minutes in the center of the preheated oven until the shells puff up and are golden brown. Do not open the oven door until at least 22 to 25 minutes have passed.
Remove the shells from the oven and let them cool completely in an area free from drafts. Cooling them inside a switched-off oven with the door open is a good option.
For the craquelin: Place the butter, salt, and sugar in a bowl and cream them together until fluffy.
Add the flour and mix until the mixture is crumbly. Use your hands to bring the crumbs together to form a soft dough.
Split the dough into two equal portions. Roll each portion inside a ziploc bag until it is about 2 to 3 mm thick. Remove excess air and freeze both bags on a tray until completely frozen.
Once frozen, cut 1.5 to 2-inch circles from the dough, aiming for about 40 discs total. Place the discs on a parchment-lined tray and return them to the freezer until needed.
For the strawberry cream: Place the strawberries and sugar in a saucepan. Heat until the strawberries soften and the sugar dissolves. Bring the mixture to a boil.
Remove from the heat and use a stick blender to blend the strawberry mixture until smooth. Cover and chill until completely cold.
When ready to make the cream, place the water in a heat-proof bowl. Sprinkle the gelatin powder over the water, mix gently, and let it bloom for 10 to 15 minutes.
Microwave the bloomed gelatin mixture for about 10 to 20 seconds until the gelatin dissolves. Do not let it boil.
Place the whipping cream, vanilla, and confectioner’s sugar in a bowl. Whisk on medium speed. Add the dissolved gelatin while the cream is whipping.
Whip the cream until you reach a soft peak stage. Continue whipping until the cream is slightly stiffer, but not at a stiff peak stage (mid-peak).
Add the chilled, blended strawberry coulis and whisk to mix it completely with the cream. Whisk until it thickens and returns to a mid-peak stage.
Keep the cream in a piping bag fitted with a ½ inch open star tip.
For the macerated strawberry filling: Place the diced strawberries and sugar in a bowl. Mix well and let it sit in the fridge to macerate for at least 20 minutes.
To assemble, you can fill the pastry cases by inserting a piping tip into the bottom, or by cutting the cases in half.
To fill from the bottom, cut a small “x” on the bottom of the pastry case. Insert the strawberry cream piping tip into the pastry case from the bottom and squeeze the cream in until full.
To cut and fill, slice the choux pastries in half along the middle or slightly closer to the top. Fill the bottom half with strawberry cream.
Optionally, place a small amount of the drained, macerated strawberries on top of the cream, then add another swirl of strawberry cream. Place the top half of the choux pastry back on.
Dust the tops with confectioner's sugar and add a sliced strawberry for decoration if desired. Serve immediately.