In small saucepan, whisk together sugar, flour, salt, half & half and egg yolks until smooth. Heat over medium heat until mixture is thickened, about 5 minutes, stirring constantly with a spoon or rubber spatula. Remove from heat; stir in vanilla and chocolate. Continue stirring until chocolate is melted. Cool quickly. To prevent a skin from forming, brush surface with melted butter. Stir before using.