1. In a medium-sized heatproof bowl, whisk the sugar and egg yolks together. Sift the flour and cornstarch together, then add to the egg mixture, mixing until you get a smooth paste.
2. Meanwhile, in a medium saucepan, bring milk to a low boil (until milk starts to steam and bubbles form around the edges of the pot.) Remove from heat and slowly add hot milk to the egg mixture, constantly whisking to prevent curdling. Pour the egg mixture into the saucepan and cook over medium heat until boiling, whisking constantly. When it boils, whisk the mixture constantly for another 30 – 60 seconds until it becomes thick.
3. Remove from heat and immediately whisk in vanilla. Set a mesh strainer over a medium bowl and pour the cooked mixture through the strainer, using a spoon to push the mixture through. Immediately cover the surface with plastic wrap to prevent a skin from forming. Cool to room temperature. If not used immediately, refrigerate until needed, up to 3 days. Whisk or stir before using to eliminate any lumps that may have formed.
1. Bake tart shells according to directions.
2. Spoon pastry cream into baked tarts, leaving enough space at the top for a thin layer of chocolate ganache. Chill for at least 30 minutes.
1. In a small saucepan, heat cream to a low boil (just until milk starts to steam and bubbles form around the edges of the pot.) Pour over chopped chocolate in a heat-safe bowl, and let stand for 5 minutes. Add vanilla and stir until the chocolate is melted. With a spoon, gently pour and spread chocolate ganache on top of the pastry cream. Chill for at least 20 minutes.
1. Combine a generous amount of icing sugar with just enough cream to make it into the icing. Place icing into a piping bag (or a small resealable bag) and cut a fine tip. Pipe onto the top of the cold-filled tart.