Summary: Just so insanely good. That's all. This can be made up to three or four days ahead. Once it's cool, cover with plastic wrap and store in the fridge.
*Cacao nibs are bits of shell-roasted cacao beans. You can get them at Whole Foods or other specialty stores. They are optional, really.
You will need a rectangular tart pan for this.

Yield: one 13¾ x 4½-inch rectangular tart

Ingredients:

1¼ cups unbleached all purpose flour
⅓ cup powdered sugar
¼ teaspoon salt
½ cup (1 stick) chilled unsalted butter, cut into ½" cubes
1 tablespoon (or more) ice water
¾ cup sugar
¼ cup water
⅓ cup heavy whipping cream
2 tablespoon (1/4 stick) unsalted butter, cut into ½" cubes
1 teaspoon apple cider vinegar
¼ teaspoon salt
⅔ cup hazelnuts, toasted , husked, coarsely chopped
⅓ cup heavy whipping cream
1½ teaspoon instant espresso powder
4 ounces high-quality milk chocolate (such as Lindt, Perugina, or Valrhona), chopped
1 tablespoon unsalted butter
1 tablespoon cacao nibs*

Instructions:

For crust: Blend flour, powdered sugar, and salt in processor. Add butter; using on/off turns, blend until mixture resembles coarse meal. Add 1 tablespoon ice water; process just until dough begins to clump together, adding more ice water by teaspoonfuls if dry. Transfer dough to 13¾ x 4½-inch rectangular tart pan with removable bottom. Press dough onto bottom and up sides of pan. Freeze crust 20 minutes.

Meanwhile, position rack in center of oven and preheat to 375° Fahrenheit. Bake crust until golden brown and cooked through, about 30 minutes. Cool crust completely in pan on rack.

For filling: Stir sugar and ¼ cup water in heavy medium saucepan over medium heat until sugar dissolves. Increase heat and boil without stirring until syrup is medium amber, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 8 minutes. Remove pan from heat. Add cream (mixture will bubble up). Place saucepan over medium heat; stir until caramel bits dissolve. Add butter, vinegar, and salt; stir until butter melts. Stir in hazelnuts. Spoon filling into crust. Chill until cold and set, about 30 minutes.

For topping: Combine cream and espresso powder in small saucepan. Bring to simmer over medium-high heat, stirring occasionally, until espresso powder is dissolved. Remove from heat. Add chocolate and butter; stir until smooth. Spread chocolate mixture over caramel. Sprinkle with cacao nibs.

Chill tart until topping is set, about 1 hour.