Prepare the Tartlet Shells: In a bowl, combine flour and powdered sugar. Cut in the cold butter until the mixture resembles coarse crumbs. Add the egg yolk and cold water, mixing until a dough forms. Flatten the dough into a disk, wrap it in plastic, and chill for 30 minutes. Preheat the oven to 350°F (175°C). Roll out the dough on a floured surface and cut into circles to fit tartlet pans. Press the dough into the pans, trim the edges, and prick the bottoms with a fork. Bake for 12-15 minutes or until golden. Cool completely.

Make the Vanilla Cream: In a saucepan, heat milk, cream, and vanilla until just simmering. In a bowl, whisk sugar, cornstarch, and egg yolks until smooth. Gradually pour the hot milk mixture into the egg mixture, whisking constantly. Return the mixture to the pan and cook over medium heat, stirring until thickened. Remove from heat, stir in butter, and let cool.

Assemble the Tartlets: Fill each tartlet shell with vanilla cream. Top with assorted fresh berries. Garnish with mint leaves.