In a saucepan, off the heat, combine the sugar, flour, and cornstarch. Add the eggs and whisk until smooth. Gradually stir in the milk. Bring to a boil over medium heat, whisking constantly and making sure to scrape the bottom and sides of the pan. Simmer gently for about 1 minute. Pour over the chocolate and add the vanilla. Combine with a whisk until the chocolate has melted. Place plastic wrap directly on the hot pastry cream. Let cool and refrigerate until the cream is completely chilled.
Puff Pastry