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

For the Tart Shells:

1. Preheat Oven: Preheat your oven to 350°F (175°C).
2. Mix Dry Ingredients: In a mixing bowl, whisk together flour, cocoa powder, and powdered sugar.
3. Add Butter: Cut in the softened butter until the mixture resembles coarse crumbs.
4. Add Egg Yolks: Stir in the egg yolk and cold water, mixing until the dough comes together.
5. Chill Dough: Wrap the dough in plastic wrap and refrigerate for about 30 minutes.
6. Roll Out Dough: Roll out the chilled dough on a lightly floured surface to about 1/8 inch thick.
7. Cut and Fit Shells: Cut out circles and fit them into a tartlet pan or mini muffin tin. Prick the bottoms with a fork to prevent bubbling.
8. Bake: Bake for 12-15 minutes until set. Allow to cool completely before filling.

For the White Chocolate Mousse:

1. Melt White Chocolate: Place the chopped white chocolate in a heatproof bowl over simmering water. Stir until melted and smooth. Remove from heat and let cool slightly.
2. Whip Cream: In a separate bowl, whip 1 cup of heavy cream until soft peaks form. Set aside.
3. Beat Egg Whites: In another bowl, beat the egg whites until foamy. Gradually add granulated sugar, continuing to beat until stiff peaks form.
4. Combine Mixtures: Gently fold the melted white chocolate into the whipped cream. Then, fold in the beaten egg whites until no white streaks remain.
5. Fill Tart Shells: Spoon the mousse into the cooled tart shells, smoothing the tops.
6. Chill Mousse: Refrigerate the filled tartlets for at least 2 hours until set.