In a small saucepan, combine the brown sugar, cornstarch and salt. Add milk and egg yolks; stir until smooth.
Cook and stir over medium heat until mixture comes to a boil. Cook and stir 1-2 minutes longer or until thickened.
Remove from the heat; stir in butter and vanilla.
Cool to room temperature, stirring several times. Pour into four individual dessert dishes. Cover and refrigerate for 1-2 hours or until chilled. Garnish with whipped topping and serve.