Bring cream to simmer in heavy large saucepan; remove from heat.
Add chocolate; let stand 1 minute.
Whisk until mixture is smooth.
Whisk in extract. Freeze until firm, about 4 hours, or chill overnight.
Line baking sheet with foil. Drop scant 1 tablespoon chocolate mixture for each of 16 truffles onto prepared sheet. Dust hands with cocoa; roll chocolate mounds into rounds. Cover and freeze truffles. Cover and chill remaining chocolate mixture for sauce. (Truffles and sauce can be made 2 days ahead.)
Line 8 1/2x4 1/2x2 1/2-inch metal loaf pan with plastic wrap, leaving long overhang.
Spread 1/3 of ice cream (about 1 2/3 cups) over bottom of prepared pan. Press 8 truffles in random pattern (and spaced apart) into ice cream layer.
Spread 1/2 of remaining ice cream over. Press remaining 8 truffles in random pattern (and spaced apart) into second ice cream layer.
Spread remaining ice cream over. Cover terrine with plastic wrap overhang. Freeze at least 6 hours and up to 2 days.
Bring sugar and 1/3 cup water to boil in small saucepan, stirring until sugar dissolves.
Pour syrup into blender; cool 10 minutes.
Add mint leaves to syrup and puree until smooth.
Transfer sauce to bowl; cool. (Can be made 1 day ahead. Cover and chill.
Whisk to blend before using.)
Stir chocolate sauce over low heat until warm.
Transfer to pitcher. Turn ice cream terrine out onto platter; peel off plastic.
Cut terrine crosswise into slices; arrange on plates.
Drizzle chocolate sauce and mint sauce around terrine.
Test-kitchen tip:Make quick work of softening the ice cream by microwaving it in ten-second intervals on the lowest power setting.
