In a medium white ceramic mixing bowl, whisk the 5 egg yolks with 1/4 cup of the sugar and the cornstarch until the mixture turns pale and ribbons off the whisk.

In a cream enameled saucepan, warm 2 cups whole milk with the vanilla bean or extract over medium heat until steam rises and small bubbles ring the edge, about 5 minutes; do not let it boil. Slowly pour the hot milk into the egg mixture in a thin stream, whisking constantly, then pour everything back into the saucepan.

Set the saucepan over medium-low heat and cook the pastry cream, whisking constantly, until it thickens to the texture of pudding and coats the back of a spoon, 3 to 4 minutes. Remove from the heat and whisk in the 2 tablespoons butter until it melts in completely.

Pour the pastry cream through a fine mesh sieve into a clean white ceramic bowl to catch any lumps. Press plastic wrap directly onto the surface and refrigerate for at least 2 hours, until fully cold and set.

While the pastry cream chills, blend the 4 eggs, remaining 2 cups milk, flour, remaining 1/4 cup sugar, salt, and the 3 tablespoons melted butter in a blender until completely smooth, about 20 seconds. Pour the batter into a glass pitcher, cover, and refrigerate for 30 minutes to let the bubbles settle.

Heat an 8 inch nonstick crepe pan over medium heat and brush lightly with melted butter. Pour in a scant 1/4 cup of batter, immediately tilting and swirling the pan so it coats the bottom in one thin, even layer.

Cook the crepe for 60 to 75 seconds, until the top looks dry and the edges lift away from the pan on their own. Flip with a thin spatula and cook the second side for about 20 seconds, just to set it, then slide the crepe onto a plate.

Repeat the pouring, swirling and flipping with the remaining batter, stacking the finished crepes on the plate as you go, until you have 20 crepes. Let the stack cool to room temperature, about 20 minutes, before filling it.

Place the first crepe, best side down, in the center of a flat white ceramic cake stand. Spread 3 tablespoons of the chilled pastry cream over it in a thin, even layer, all the way to the edge.

Lay a second crepe on top and press gently to settle it, then spread with another 3 tablespoons of pastry cream. Repeat the layering with the remaining crepes and cream, finishing with your best looking crepe on top, left bare.

Cover the cake loosely and refrigerate overnight, or for at least 8 hours, so the layers set firm enough to slice cleanly.

Just before serving, dust the top of the cake generously with powdered sugar and scatter the raspberries and blueberries over the center. Slice with a sharp knife, wiping the blade clean between cuts, so each slice shows its layers.