Blend flour, powdered sugar, and salt in processor. Add butter. Pulse 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 tart pan. Press dough onto bottom and up sides of pan. Freeze crust 20 minutes.

Meanwhile, position rack in center of oven and preheat to 375°F. Prick the crust thoroughly with a fork. Line the crust interior with foil and weight with pie weights or dried beans. Bake crust until golden brown and cooked through, about 30 minutes. Cool crust completely in pan on rack.

Stir sugar and 1/4 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 pecans. Spoon filling into crust. Chill until cold and set, about 30 minutes.

In a medium bowl, heat the cream in the microwave until it begins to bubble. Add espresso powder and stir until completely dissolved. Stir in the Kahlua (if using). Add chocolate and butter and let sit until the chocolate begins to melt, then stir until smooth. Spread chocolate mixture over caramel. Sprinkle with cacao nibs. Chill tart until topping is set, about 1 hour.

Remove tart pan sides. Place tart on platter; cut crosswise into 8 bars and serve.

Can be made 2 days ahead. Cover and keep chilled.