In small bowl, beat egg yolks with whisk. Gradually beat about 1/2 cup hot mixture into egg yolks. Gradually stir yolk mixture into hot mixture in saucepan. Cook over medium heat, stirring constantly, until mixture starts to gently boil. Cook 2 minutes longer, stirring constantly. Remove from heat; stir in 3 tablespoons butter. Pour over cooled baked crust. Cool at room temperature 15 minutes, then cover filling with plastic wrap. Refrigerate at least 3 hours until set.