Combine the egg whites, sugar and salt in a metal bowl over a pan of water (don't let the water touch the bottom of the bowl) or in the top of a double boiler. Attach a candy thermometer to the side of the pan and cook over medium heat, whisking constantly, until the mixture reaches 140 degrees.

Transfer mixture to the bowl of a mixer and with the whisk attachment, beat on high speed for 20 - 25 minutes until the meringue is cool.

While the meringue is cooling, combine the chocolate and the butter in a metal bowl set over the same pan of water (or in the top of a double boiler) and cook over medium heat until the chocolate and butter are completely melted and smooth. Remove from the heat and stir in the dulce de leche. Set aside.

In another bowl, combine the whipping cream and the rum and whip the cream until soft peaks form. Cover and refrigerate until needed.

If serving as a sliced dessert, line a 9 x 5 inch loaf pan with plastic wrap and set aside.

Pour the chocolate mixture into a large bowl and stir in the dulce de leche. Fold in the whipped cream. Then fold in ¼ of the meringue mixture and then fold in the remainder of the meringue. If some meringue streaks remain, that is ok! Scrape the semifreddo into the prepared loaf pan or into a plastic container. Cover and freeze overnight.

To serve, remove the plastic and, if using the loaf pan, invert the loaf pan on a platter and peel away the plastic wrap and slice into ½ inch thick slices. If using a plastic container, use an ice cream scoop to scoop portions into dessert cups or glasses. Garnish each with chocolate shavings if desired.