Mix brown sugar, cornstarch and salt in a medium saucepan. Slowly add milk, stir until smooth. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute. Stir at least 1/2 of the mixture into the egg yolks, whisking as you incorporate the hot mixture into the egg yolks. Blend egg mixture into the saucepan. Boil and stir 1 minute. Remove from heat. Stir in butter and vanilla. Pour into dessert dishes. Cool slightly, refrigerate.