Preheat the oven to 180°C (350°F). Combine flour, cold butter, and sugar in a bowl. Mix until the mixture resembles coarse crumbs. Add cold water and knead into a dough. Wrap in plastic wrap and chill for 15 minutes.

Roll out the dough and press it into a small tart pan. Prick the bottom with a fork and bake for 15 minutes or until golden. Let cool.

In a saucepan, heat the sugar over medium heat until melted and golden brown. Add butter and stir until combined. Slowly pour in the heavy cream, whisking constantly. Remove from heat and let it cool slightly.

Pour the caramel filling into the cooled tart crust. Spread evenly with a spatula. Sprinkle toasted hazelnuts over the caramel.

Chill the tart in the refrigerator for at least 1 hour before slicing and serving.