Creating these Elegant White Chocolate Mousse Tartlets is simple if you follow these steps:

Prepare the Tart Shells

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Mix the Dough: In a mixing bowl, combine the flour and powdered sugar. Add the softened butter and mix until crumbly.
3. Add Egg Yolk: Incorporate the egg yolk and mix until the dough starts to come together. If it’s too dry, add cold water, one tablespoon at a time, until it forms a ball.
4. Chill the Dough: Wrap the dough in plastic wrap and refrigerate for about 30 minutes.
5. Roll Out the Dough: On a floured surface, roll out the chilled dough to about 1/8-inch thickness. Cut into circles to fit your tartlet pans.
6. Place in Tart Pans: Press the dough circles into the tartlet pans. Prick the bottoms with a fork to prevent bubbling.
7. Bake: Bake in the preheated oven for 15-20 minutes or until lightly golden. Remove from the oven and let cool completely.

Prepare the White Chocolate Mousse

8. Melt the Chocolate: In a heatproof bowl, melt the chopped white chocolate over a double boiler or in the microwave in short bursts, stirring until smooth. Set aside to cool slightly.
9. Whip the Cream: In a separate bowl, whip the heavy cream until soft peaks form. Be careful not to overwhip.
10. Beat Egg Whites: In another bowl, beat the egg whites until frothy. Gradually add the granulated sugar and continue beating until stiff peaks form.
11. Combine Mixtures: Gently fold the melted white chocolate into the whipped cream. Then, carefully fold in the beaten egg whites until fully incorporated.
12. Chill the Mousse: Spoon the mousse into the cooled tart shells and smooth the tops. Refrigerate for at least one hour to set.