Spread a 1/2-inch-thick layer of ice cream into bottom and up sides of a plastic wrap-lined 2-quart bowl. Cover and freeze 2 hours.
Drain cherries and pineapple; chop cherries. Pat fruit dry with paper towels.
Combine fruit, 1 cup pecans, and vanilla, tossing to coat.
Beat 1 cup whipping cream and 1 tablespoon sugar at high speed with an electric mixer until stiff peaks form. Fold in fruit mixture; spread evenly over ice cream layer. Cover and freeze 8 hours.
Remove from freezer; let stand 20 minutes. Invert Cassata onto a cold serving dish. Let stand 10 more minutes.
Beat remaining 1 cup whipping cream and remaining 1 tablespoon sugar until stiff peaks form. Drizzle Cassata with chocolate sauce; dollop with whipped cream. Garnish, if desired, and serve immediately.
Note: If preparing ahead, remove Cassata from bowl, and return to freezer until ready to serve. Let stand 30 minutes before serving. Top with chocolate sauce and whipped cream; garnish, if desired.
Prep: 20 min., Freeze: 10 hrs, Stand: 30 min.