For pastry cream, in a small saucepan, combine sugar and cornstarch. Stir in milk until smooth. Cook and stir over medium heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. , Remove from the heat. Gently stir in vanilla. Cool to room temperature without stirring. For buttercream, i n a bowl cream butter until fluffy, about 5 minutes. Gradually beat in cooled pastry cream. If desired, beat in confectioners' sugar and additional vanilla extract until fluffy, about 5 minutes. If necessary, refrigerate until frosting reaches spreading consistency. Store in the refrigerator.