Melt the white chocolate
Place the chopped white chocolate in a heatproof bowl and melt gently over a double boiler or in short microwave bursts, stirring until smooth and glossy. Let it cool slightly.

Whip the cream
In a separate bowl, whip the cold heavy cream until soft to medium peaks form. The cream should hold shape but still look smooth and airy.

Combine gently
Stir the vanilla extract and salt into the melted white chocolate. Add one spoonful of whipped cream to loosen the chocolate, then gently fold in the remaining cream until smooth and fluffy.

Fill the tartlets
Spoon or pipe the white chocolate mousse into the tartlet shells, smoothing the tops if desired.

Chill to set
Refrigerate the tartlets until the mousse is firm but creamy.

Garnish and serve
Top with fresh fruit, chocolate shavings, or nuts just before serving.