In a small saucepan, combine the sugar, cornstarch and salt. Stir in milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to low; cook and stir for 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 for 2 minutes. Remove from the heat; gently stir in vanilla.
Cover and chill for 1 hour. Just before serving, fold in banana. Garnish with mint if desired.