Prepare the tart shell. Set aside.

Prepare the cream filling:

In a saucepan set over moderate heat, whisk the egg yolks. Gradually whisk in the sugar in a thin stream. (Gradually adding the sugar assures the eggs will not become grainy.)

Whisk until pale yellow in color and ribbons form when dropped from the whisk.

In a separate bowl, sift together the flour and cornstarch. Beat into the egg mixture.

Gradually pour the hot milk into the egg mixture, stirring with a wooden spoon. Scrape the sides and bottom of the pan as the sauce cooks. Do not let it boil as it thickens. When it coats a spoon well and leaves a trail when you draw a finger across the surface of the spoon, the pastry cream is ready.

(If the cream is overcooked the eggs will scramble.)

Immediately remove from heat. Pour through a sieve into a bowl and set over ice bath for about 5 minutes. The ice bath will stop the cooking. Stir in the butter. And then the vanilla. Cover the surface with plastic wrap. Refrigerate until set, about 2 hours.

To assemble the tart:

Pour the pastry cream evenly into the prepared tart shell. Arrange the blueberries and raspberries.

Refrigerate until ready to serve.