Preheat oven to 350°F (180°C). Combine flour, butter, sugar and salt in food processor; pulse until crumbly.
Firmly press dough into 9 x 13-inch (3 L) baking pan lined with foil that hangs over the edges. Bake for 25 minutes or until lightly browned around the edges; cool slightly.
Stir milk with sugar, cornstarch, cocoa powder and salt in medium saucepan until combined. Cook set over medium heat, stirring constantly, for 7 minutes or until thickened.
Whisk some of the hot mixture into the egg yolks, while whisking vigorously. Pour egg mixture back into saucepan with remaining milk mixture. Cook, stirring constantly for 1 minute.
Remove from heat; stir in chocolate and butter until smooth. Pour over baked crust; smooth top. Set aside.
Place half the water in small bowl and sprinkle gelatin over top; set aside for 5 minutes. Meanwhile, combine cranberries, sugar and remaining water in medium saucepan set over medium heat. Bring to a boil and simmer for 3 to 5 minutes or just until cranberries are softened but still whole.
Strain cranberry mixture. Stir hot syrup with gelatin mixture until smooth; stir cranberries back into syrup. Cool for 20 minutes.
Spread cranberry mixture over chocolate. Sprinkle pistachios over top. Chill for at least 6 hours or overnight. Use foil to carefully transfer the tart from pan to a cutting board. Cut into 24 portions.