For the crust:
Place the flour, coconut flakes, and butter in a food processor, fitted with a pastry blade, and pulse a few times to combine the ingredients.
Add the water and pulse several times until the dough forms a ball.
On a floured surface, roll out the dough and place it in buttered tartlet molds.
Poke the bottom with a fork and place in the freezer for 30 minutes.
Preheat the oven to 375°F (190°C).
Cover the dough with aluminum foil and fill the cavities with dried beans.
Bake the tartlets for 25 minutes, then remove the beans and foil and continue baking for 8 minutes.
Remove from the oven and let cool completely.

For the filling:
Bring the milk to a boil.
Meanwhile, whisk the egg yolks with the sugar, cornstarch, and salt until smooth. Gradually add the hot milk, whisking constantly, then return the mixture to the saucepan.
Without stopping whisking, bring to a boil until thickened.
Pour the mixture over the chopped chocolate and let it sit for 2 minutes before whisking until the chocolate is melted.
Add the butter and whisk until well combined.

Pour the mixture into the reserved tartlet shells and let cool completely.

For the whipped cream:
Beat the cream until it starts to thicken. Add the powdered sugar and vanilla extract and continue beating until stiff peaks form.
Top the tartlets with the whipped cream.