Creating these Elegant White Chocolate Mousse Tartlets can be straightforward if you follow these simple steps:

For the Tartlet Shells:

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Prepare the Dough: In a mixing bowl, combine the flour and powdered sugar. Add the softened butter and mix until crumbly. Stir in the egg yolk and cold water until the dough forms.
3. Chill the Dough: Wrap the dough in plastic wrap and refrigerate for 15-20 minutes.
4. Shape the Shells: On a lightly floured surface, roll out the dough to about ¼ inch thick. Cut into circles and press into a tartlet pan. Prick the bottoms with a fork.
5. Bake: Bake the tartlet shells in the preheated oven for 15-20 minutes or until golden brown. Allow them to cool completely.

For the White Chocolate Mousse:

6. Melt White Chocolate: In a double boiler, melt the chopped white chocolate until smooth. Remove from heat and let it cool slightly.
7. Whip the Cream: In a separate bowl, whip the heavy cream until soft peaks form. Gently fold the melted white chocolate into the whipped cream.
8. Beat Egg Whites: In another bowl, beat the egg whites until foamy. Gradually add the granulated sugar and continue beating until stiff peaks form.
9. Combine Mixtures: Gently fold the beaten egg whites into the white chocolate mixture until fully combined.
10. Fill Tartlet Shells: Spoon or pipe the mousse into the cooled tartlet shells. Smooth the tops and refrigerate for at least 2 hours to set.