Combine sugar, cornstarch, and salt in a bowl. Pour milk into a medium saucepan. Add dry ingredients to milk and bring to a boil over medium heat, stirring constantly, until mixture thickens. Lower heat and simmer, stirring one more time. Whisk half of the milk mixture into egg yolks, then add mixture to saucepan. Simmer, stirring occasionally, for 1 minute. Remove from heat.