How to Make It
In a food processor, pulse the flour with the confectioners' sugar and salt. Add the butter and egg yolk and process until a soft, crumbly dough forms. Transfer the dough to a 9 1/2-inch fluted tart pan with a removable bottom. Pat the dough over the bottom and up the side of the pan in an even layer. Refrigerate until firm.
Preheat the oven to 350°. Line the tart shell with parchment paper and fill it with pie weights or dried beans. Bake the shell for 30 minutes, or until golden around the edge and dry in the center. Remove the parchment and weights and cover the rim with foil. Continue to bake the shell for 15 to 20 minutes longer, or until golden and cooked through. Transfer to a rack; let cool.
In a small saucepan, heat the cream with the milk over moderate heat until small bubbles appear around the edges. Off the heat, add the chocolate and let stand for 1 minute, then whisk until smooth. Let cool for 5 minutes. Whisk in the egg; the mixture will thicken slightly.
Set the tart shell on a baking sheet and fill it with the chocolate custard. Bake for 25 minutes, or until set around the edges, but still very jiggly in the center. Transfer the tart to a rack to cool, then refrigerate until chilled.
In a very small saucepan, sprinkle the gelatin over the water and let stand until softened, about 5 minutes. Add the coffee extract and cook over low heat just until the gelatin melts; let cool slightly.
In a large bowl, beat the cream, mascarpone and sugar with an electric mixer until firm peaks form. At low speed, scrape the gelatin into the bowl and beat to combine. Dollop the cream onto the tart and swirl decoratively. Sift the cocoa over the cream. Refrigerate the tart until firm before serving.
If coffee extract is unavailable, dissolve 1 1/2 teaspoons of espresso powder in 1 1/2 teaspoons of water.