Cut vanilla bean in half lengthwise and boil with 400 ml (approximately 1 2/3 cup) milk. Combine yolks, sugar, remaining milk and cornstarch. Remove vanilla pod from milk and slowly add milk, stirring constantly, to yolk mixture.
Continue to whisk until creamy. Pour into a mold lined with plastic wrap and chill for 2 hours. Remove, release from pan and discard plastic wrap. Serve garnished with mint leaves.