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

Prepare the Tart Shells

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Mix Dry Ingredients: In a large bowl, whisk together the flour, cocoa powder, and powdered sugar.
3. Add Butter: Add the softened butter to the dry ingredients and mix until crumbly.
4. Add Egg Yolk: Mix in the egg yolk until the mixture forms a dough.
5. Add Water: If necessary, add cold water, one tablespoon at a time, until the dough holds together.
6. Chill Dough: Wrap the dough in plastic wrap and refrigerate for 30 minutes.
7. Roll Out Dough: On a floured surface, roll out the chilled dough to about 1/8-inch thickness.
8. Cut and Press Dough: Cut out circles to fit your tartlet pans. Press the dough into the pans, trimming any excess.
9. Blind Bake: Prick the bottoms with a fork and bake for 15-20 minutes or until firm. Allow to cool completely.

Prepare the White Chocolate Mousse

10. Melt White Chocolate: In a heatproof bowl, melt the white chocolate over a pot of simmering water, stirring until smooth. Remove from heat and let cool slightly.
11. Whip Cream: In a separate bowl, whip 1 cup of heavy cream until soft peaks form. Set aside.
12. Whip Egg Whites: In another bowl, beat the egg whites until foamy. Gradually add the sugar and whip until stiff peaks form.
13. Combine Ingredients: Gently fold the melted white chocolate into the whipped cream until just combined. Then, fold in the egg whites until no white streaks remain.
14. Chill Mousse: Spoon the mousse into the cooled tart shells and smooth the tops. Refrigerate for at least 2 hours to set.