« » Make the dough: Pulse flour, cocoa, sugar, and salt in a food processor until combined. Add butter, and pulse until mixture resembles coarse meal, with a few pea-size pieces remaining. With the machine running, add water in a slow, steady stream, until dough just holds together. (Dough should not be sticky.)
On a lightly floured work surface, shape dough into 2 disks, and wrap each in plastic. Refrigerate until firm, at least 30 minutes (or up to 3 days).
Make the filling: Bring cream, sugar, mint, and salt to a simmer in a medium saucepan. Remove from heat, and let cool completely, about 30 minutes.
Meanwhile, preheat oven to 350 degrees. On a lightly floured surface, roll 1 disk of dough to a 16-by-6-inch rectangle, 1/4 inch thick. Press dough into bottom and up sides of a 13 1/2-by-4 1/2-inch rectangular fluted tart pan with a removable bottom. Using a sharp paring knife, trim excess dough flush with the edges of pan. Repeat with remaining disk of dough. Refrigerate for 15 minutes.
Line dough with parchment, and fill with pie weights or dried beans. Bake until edges are dry, about 20 minutes. Remove weights and parchment. Bake until just darkened, 10 to 15 minutes. Let cool completely in pans on wire racks.
With a pastry brush, coat the bottom of each tart shell with an even layer of melted chocolate. Refrigerate until chocolate sets, about 10 minutes.
Sprinkle gelatin over water in a small bowl. Let stand until softened.
Remove mint from cream mixture, and bring cream mixture to a simmer. Whisk in softened gelatin until it dissolves. Remove from heat, and stir in yogurt.
Divide cream mixture between tart shells. Carefully transfer to the refrigerator. Chill, uncovered, until set, at least 4 hours (or overnight).
Carefully unmold tarts. Top 1 tart with red currants and 1 with strawberries. Garnish both tarts with mint stems. Slice and serve immediately.
