In a saucepan combine sugar and cornstarch. Stir in half-and-half. Cook, stirring, over medium heat until mixture thickens. In a small bowl beat egg yolks. Gradually add half of cream mixture to yolks, stirring constantly. Return egg mixture to saucepan and heat just to simmering. Reduce heat and simmer 2 minutes more, stirring all the while. Remove from heat and stir in butter. Pour custard into 4 small ramekins, about 1/2 cup capacity. Top each with vanilla wafers and raspberries. Cover with plastic wrap and refrigerate until ready to serve. Garnish with mint leaves.