Elegant white chocolate mousse sits atop crisp chocolate tart shells in these bite sized sweet treats that your guests will adore.
Process flour, cocoa powder, sugar, and salt in a food processor until combined. Add butter; pulse until mixture resembles coarse meal. With machine running, add eggs, mixing just until dough starts to come together. Turn out dough onto a lightly floured work surface, and knead into a ball. Wrap in plastic wrap. Refrigerate 30 minutes.
Roll out dough on a lightly floured surface to a 1/8-inch thickness. Line twenty-four 2 1/2-inch fluted tart pans and twenty-four 3 1/4-by-1 3/4-inch straight-sided tart pans, rerolling scraps as needed. Transfer to a baking sheet; refrigerate until firm, about 30 minutes.
Preheat oven to 350 degrees. Prick bottoms with a fork. Bake until firm and dry, 15 to 18 minutes. Let cool completely.
Put 1/4 cup cold water in a small bowl; sprinkle with gelatin. Let soften about 5 minutes. Meanwhile, bring 3/4 cup cream to a boil in a small saucepan over medium-high heat. Remove from heat. Add gelatin mixture, stirring for 30 seconds to dissolve completely.
Put chocolate in the bowl of a food processor. With machine running, pour in gelatin mixture; process until smooth. Transfer to a medium bowl, and cover. Refrigerate until almost consistency of pudding, about 30 minutes; stir until smooth.
Whisk remaining 1 3/4 cups cream in a large bowl just until stiff peaks form. Fold into chocolate mixture until combined.
Put mousse in a pastry bag fitted with a small open-star tip (such as Ateco #22). Pipe into cooled tart shells.