Combine sugar, cornstarch, and egg in a bowl; stir well with a wire whisk, and set aside.
Heat milk over medium-high heat in a medium heavy saucepan to 180° or until tiny bubbles form around edge (do not boil). Remove from heat. Gradually add hot milk to sugar mixture, stirring constantly with whisk. Return milk mixture to pan. Add margarine, and cook over medium heat 5 minutes or until thickened and bubbly, stirring constantly. Reduce heat to low, and cook an additional 2 minutes, stirring constantly. Remove from heat; stir in vanilla. Pour into a bowl; cover surface with plastic wrap, and let cool completely. Uncover and gently fold in 1 cup whipped topping; cover and chill at least 2 hours.
Split ladyfingers in half lengthwise. Line each of 8 dessert compotes with 5 ladyfinger halves, standing upright. Spoon 1/2 cup custard into center of each compote. Melt preserves, and drizzle over custard. Top each serving with 1 tablespoon whipped topping. Garnish with mint leaves, if desired.