Mousse base:

5 large egg yolks

3 tablespoons granulated sugar

Pinch of salt

1 1/2 cups milk

1 teaspoon vanilla extract

Dark chocolate layer:

2 1/2 ounces bittersweet chocolate, finely chopped

1/3 mousse base (generous 1/2 cup; see above)

1/2 cup heavy cream

1 tablespoon granulated sugar

White chocolate layer:

7 1/2 ounces white chocolate, finely chopped

1/3 mousse base (generous 1/2 cup; see above)

1/2 cup heavy cream

1 tablespoon granulated sugar

Milk chocolate layer:

3 ounces milk chocolate, finely chopped

1/3 cup mousse base (generous 1/2 cup; see above)

1/2 cup heavy cream

1 tablespoon granulated sugar

First make the mousse base, which is used in each of the chocolate layers. In a medium-size bowl that fits snugly into a pot, as for a double boiler, whisk together the egg yolks, sugar, and salt; set aside. Put some water in the pot, making sure the bottom of the bowl does not touch the water, and bring to a gentle boil.

Scald the milk in a saucepan, heating it just until little bubbles appear around the sides; do not boil. Slowly whisk the milk into the egg yolk mixture. Set the bowl into the pot of gently boiling water, and cook the mixture, stirring all the time, until the custard thickens slightly, 3 to 5 minutes. Watch closely. Stir in the vanilla, strain the mousse base through a sieve into a clean bowl, and chill thoroughly.

Line a loaf pan, at least 6-cup capacity, with plastic wrap; set aside.

In a small bowl that fits snugly into a pot of gently boiling water, or in the top of a double boiler, melt the bittersweet chocolate over the water. When the chocolate is smooth and has cooled somewhat, slowly whisk in the mousse base. Let it cool a bit.

In a small bowl, whip the heavy cream and sugar until soft peaks form. Fold the cream into the chocolate mixture. Pour the dark mousse into the plastic-lined loaf pan, filling the corners and smoothing the surface. Freeze, making sure the pan is level.

Prepare the white and milk chocolate mousses separately in the same way as the dark chocolate mousse. Refrigerate the white and milk chocolate mousses in their separate bowls.

When the dark chocolate mousse is firm, in about 30 minutes, spread the white chocolate mousse on top evenly. When firm, in about 45 minutes, spread the milk chocolate mousse on top. Cover tightly and freeze hard, about 4 hours.

To serve, take the loaf pan out of the freezer and leave it at room temperature for a few minutes to soften a bit. Lift out the semifreddo, unmold it on a board, and unwrap it. Cut the semifreddo into thin slices, as it is rich. Spoon a little of the sauce around and serve at once.

Serves 6 to 8.