Combine flour, salt, and sugar in a large bowl, then cut in chilled butter until resembling coarse breadcrumbs. Add egg and mix gently. Shape dough into a disc, wrap in plastic, and chill for at least 1 hour.

Preheat oven to 190°C (374°F). Roll out dough to about 1/4 inch thickness, fit into tartlet tins, and blind bake for 10 minutes. Remove weights, bake for an additional 5 minutes until golden. Cool on wire racks.

Whisk together egg yolks and sugar until pale. In a separate saucepan, warm heavy cream over low heat, then pour in egg mixture, stirring until thickened, about 5 minutes. Remove from heat, add chopped chocolate, and stir until smooth. Cool completely.

Whip remaining heavy cream until medium-stiff peaks form. Gently fold into the cooled chocolate mixture. Chill the filling for at least 30 minutes.

Spoon mousse into cooled tart shells, smooth the tops, cover with plastic wrap, and refrigerate for at least 4 hours or overnight. Garnish with berries or chocolate ganache before serving.