Make the Tart Shells
Preheat oven to 350°F (175°C).
Mix flour, sugar, and salt in a bowl.
Cut in butter until mixture resembles crumbs.
Add egg yolk and water, mixing until dough forms.
Roll dough and press into tartlet molds.
Poke holes in the crust with a fork.
Bake for 15-20 minutes until golden.
Let cool before filling.
Prepare the Coffee Mousse
Whip heavy cream, powdered sugar, and espresso powder to stiff peaks.
Stir in vanilla extract and chill for 15 minutes.
Assemble the Tartlets
Pipe coffee mousse into the cooled tart shells.
Drizzle melted chocolate over the top.
Let set for 10 minutes before serving.