In a bowl, whisk the egg yolks, sugar, flour and cornstarch. In a saucepan, bring the milk and cream to a simmer; whisk 1/2 cup of the mixture into the eggs, then whisk the egg-milk mixture into the saucepan. Cook the pastry cream over low heat, whisking constantly, until thick, 2 minutes. Off the heat, whisk in the dark chocolate until melted. Transfer to a bowl, cover with plastic wrap and refrigerate until cool, about 2 hours.