1. Prepare the Tartlet Shells

• In a mixing bowl, whip the softened butter and sugar until light and fluffy.
• Add the egg, vanilla sugar, and a pinch of salt. Mix well until fully incorporated.
• Gradually add the flour and mix until the dough comes together.
• Wrap the dough in plastic wrap and refrigerate for about 30 minutes to firm up.

2. Shape and Bake the Tartlet Shells

• Preheat your oven to 180°C (350°F).
• Roll out the chilled dough on a lightly floured surface to a thickness of 3-4mm.
• Cut circles from the dough using a cookie cutter or glass to fit the size of your tartlet pans.
• Press the dough circles into the pans, trimming any excess, and prick the bottoms with a fork.
• Bake the tartlet shells for 10-12 minutes, or until golden brown. Allow them to cool completely before filling.

3. Make the Chocolate Filling

• In a saucepan, whisk together the egg yolk, sugar, corn starch, and vanilla sugar until smooth.
• Gradually add the milk while whisking continuously to prevent lumps.
• Cook over medium heat, stirring constantly, until the mixture thickens to a custard-like consistency.
• Remove from heat and stir in the chopped dark chocolate and butter until smooth and glossy.

4. Assemble the Tartlets

• Once the tartlet shells are completely cool, fill each shell generously with the chocolate custard.
• Smooth the tops with a spatula or the back of a spoon.

5. Chill and Serve

• Refrigerate the tartlets for at least 1 hour to allow the filling to set.
• Garnish with cocoa powder, melted chocolate drizzle, fresh berries, or sea salt before serving.