Make the crust: Combine flour, sugar, and salt in a food processor. Add butter and pulse until mixture resembles coarse crumbs. Mix in egg yolk until dough forms. Wrap in plastic; chill for 30 minutes.
Roll out dough on a floured surface and press into a 9-inch tart pan; trim excess. Freeze for 10 minutes, then bake with pie weights at 375°F (190°C) for 20 minutes. Remove weights; cool.
Prepare the filling: Heat cream and sugar in a saucepan until just simmering. Pour over chocolate in a bowl; let stand 1 minute, then stir until smooth. Whisk in eggs, egg yolk, and butter.
Pour filling into the cooled crust. Bake at 325°F (165°C) for 15-20 minutes until set but still slightly wobbly in the center. Cool completely, then chill.
Serve at room temperature with suggested toppings.
Notes
Use high-quality chocolate for the best flavor.
• Chill the dough for the crust before rolling it out to make it easier to work with.
• Bake the crust 'blind' (empty, with weights) to prevent puffing and ensure crispiness.
• Let the tart cool completely before slicing to allow the filling to set.
• Serve at room temperature to best enjoy the flavors.