In a food processor, pulse together the flour and powdered sugar. Add the cold, cubed butter and pulse until the mixture resembles fine breadcrumbs. Add the egg yolk and one tablespoon of cold water, pulsing until the dough just comes together. If needed, add the second tablespoon of water. Turn the dough onto a lightly floured surface, knead briefly to bring it together, then wrap in plastic wrap and refrigerate for 30 minutes.

Preheat the oven to 350°F (175°C). Roll the chilled dough out to about ⅛ inch thickness. Cut into rounds and press into mini tart pans or a muffin tin. Prick the bottoms with a fork and bake for 12–15 minutes, or until lightly golden. Let cool completely before filling.

In a heatproof bowl, melt the white chocolate over a pan of gently simmering water or in short bursts in the microwave. Let it cool slightly.

In a separate bowl, beat the softened cream cheese with vanilla extract until smooth. In another bowl, whip the heavy cream until soft peaks form.

Stir the melted white chocolate into the cream cheese mixture, then gently fold in the whipped cream until smooth and fluffy.

Spoon or pipe the mousse into the cooled tart shells. Smooth the tops or swirl them for texture. Chill in the refrigerator for at least 1 hour to allow the mousse to set.

Top each tartlet with fresh berries, white chocolate curls, or edible flowers just before serving.