In blender, purée 1½ cups of the strawberries; strain through fine-meshed strainer, pushing on strawberry purée with a spoon. Combine purée with remaining 3 cups strawberries and sugar.
Slit vanilla bean lengthwise and scrape out seeds; add bean and seeds to strawberry mixture. Let stand 10 minutes; transfer to shallow baking dish large enough to hold strawberries in one layer.
Roast in oven about 20 minutes or until strawberries are cooked and juices have reduced to a syrup.
To make Chocolate Sauce:
In saucepan, bring sugar and 3 tablespoons water to a boil; reduce heat and simmer until sugar dissolves. Add cream and chocolate. Cook over low heat, stirring, until chocolate melts. Remove from heat; add liqueur.
Spoon 2 tablespoons Chocolate Sauce in the bottom of each of 6 sundae glasses.
Add a (¾-cup) scoop ice cream; top with 2 tablespoons Roasted Strawberry Sauce. Repeat layers. Top each with ¼ cup fresh strawberries.