In a bowl, whisk the egg yolks, egg, 1/4 cup sugar, and cornstarch. In a saucepan, bring the milk and 1/4 cup sugar to a simmer; slowly pour 1/2 cup of the mixture into the eggs while whisky continuously, then whisk the egg-milk mixture into the rest of the milk in the saucepan.Cook the pastry cream over low heat, whisking constantly, until thick, about 5 minutes. Off the heat, whisk in the vanilla, dark chocolate and butter until melted. Transfer to a bowl, cover with plastic wrap and refrigerate until cool, about 2 hours.
Assemble Your Tart
Whip the cream, sugar and vanilla to soft peaks. Fill your tart shell(s) with the chocolate cream and then pile them high with the whipped cream. Garnish with chocolate shavings or chopped chocolate if desired. Enjoy!
Tip: If you are not eating your tarts right away (good luck resisting), you can brush the inside of the tart shell with melted chocolate to prevent it from getting soft