In heavy 2-quart saucepan, mix 1/3 cup sugar, 3 tablespoons cornstarch and the salt with wire whisk. Gradually beat in half-and-half, then egg yolks. Cook over medium heat about 6 minutes, stirring constantly with wire whisk, until mixture just comes to a boil and begins to thicken. Remove from heat; stir in vanilla and rum extract. Pour into medium bowl; cover with plastic wrap, touching wrap directly on surface. Refrigerate at least 2 hours but no longer than 2 days.