In a medium heavy-bottomed pot, bring the milk to a boil, and then turn off the heat. Whisk the egg yolks together in a medium bowl, and then whisk in ½ cup sugar and cornstarch. Continue whisking until the mixture thickens and is a pale-yellow color. Slowly whisk in the hot milk, at first a few tablespoons at the time, and then more quickly. Return the mixture to the stove, and cook over medium heat, alternating between the whisk and a spatula, until the pastry cream thickness to a pudding-like consistency.
Remove from the stove and stir in the butter and salt. Transfer the mixture to a bowl. Place a piece of plastic wrap on the surface to keep it from forming a skin. Poke a few holes in the plastic to let the heat escape. Cool in the refrigerator.
When the custard has cooled, fold in the crème fraiche.
Preheat the broiler. Toss the raspberries with 1 tablespoon granulated sugar, and scatter half of them on the bottom of a 9 by 9-inch gratin dish. Spoon the custard over the raspberries. Scatter the remaining raspberries on top. Sift the confectioners’ sugar over the top and pass under the broiler for about 7 minutes, until bubbling and gratineed on top.
Serve the gratin at the table with a big serving spoon.