In a mixing bowl, combine flour, salt, and sugar. Cut in the chilled butter until the mixture resembles coarse breadcrumbs, then gently mix in the egg yolks until a dough forms. Wrap the dough tightly in plastic wrap and refrigerate for at least 1 hour to ensure it firms up and prevents shrinking during baking.

Preheat your oven to 190°C (374°F). Roll out the chilled pastry dough to about ¼ inch thick. Press the dough into tartlet tins, trimming the edges. Place parchment paper over each tart shell and fill with baking weights. Bake for 10 minutes, then remove the weights and bake for another 5 minutes until golden. Cool on wire racks.

In a bowl, beat the egg yolks and sugar together until pale and creamy. In a saucepan, gently heat the heavy cream over low heat until warm. Gradually whisk the warm cream into the yolk mixture, then return to the saucepan. Cook on low heat, stirring constantly, until it thickens slightly and coats the back of a spoon. Mix in the melted white chocolate, stirring until smooth. Allow to cool completely.

In a separate bowl, whip the remaining heavy cream to medium-stiff peaks. Be careful not to overwhip, as you want to maintain the mousse's light texture. Carefully fold the whipped cream into the cooled white chocolate mixture in a gentle motion to retain airiness.

Fill the cooled tart shells with the mousse, smoothing the tops with a spatula. Cover each tart with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, allowing the mousse to set and develop its rich flavors. Just before serving, garnish with fresh berries or mint leaves.