Preheat oven to 350°F (180°C).
In bowl, stir wafer crumbs with butter until moistened. Press onto bottom and up side of 9-inch (23 cm) round or square tart pan with removable bottom. Bake in center of oven for 10 minutes or until set. Let cool completely on rack.
Caramel Filling: In heavy saucepan, combine sugar, water and corn syrup. Bring to boil; boil, without stirring, but brushing down sides with wet pastry brush, for about 8 minutes or until mixture turns caramel colour. Remove from heat; whisk in butter. Whisk in nuts and cream. Pour filling into prepared crust. Refrigerate for about 1 hour or until set.
In clean saucepan, heat cream just until boiling; pour over chocolate and butter in bowl, whisking until melted and smooth. Let cool slightly. Pour over caramel nut filling. Refrigerate for at least 8 hours or until set, or for up to 2 days.
Arrange nuts closely together on foil-lined baking sheet. In saucepan, combine sugar with water. Bring to a boil; boil, without stirring, but brushing down sides with wet pastry brush, for 5 to 7 minutes or until mixture turns caramel colour; pour over nuts; let cool completely. (Make-ahead: nut topping can be stored, uncovered, for up to 1 day). Break into pieces; finely chop in food processor.
Remove sides of tart pan; slide tart onto serving platter, removing bottom. Whip cream; spoon into pastry bag fitted with rosette tip. Pipe border of rosettes around edge of tart. Sprinkle with nut topping.