Step 1 Preheat oven to 350°F (180°C). Combine flour, butter, sugar and salt in food processor; pulse until crumbly.
Step 2 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.
Instructions (Chocolate Custard)
Step 1 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.
Step 2 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.
Step 3 Remove from heat; stir in chocolate and butter until smooth. Pour over baked crust; smooth top. Set aside.
Instructions (Cranberry Topping)
Step 1 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.
Step 2 Strain cranberry mixture. Stir hot syrup with gelatin mixture until smooth; stir cranberries back into syrup. Cool for 20 minutes.
Step 3 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.