1. Preheat the oven to 200º. Line two cookie sheets with aluminum foil, shiny-side down.
2. Using an electric mixer, whisk egg whites and salt in a large bowl (make sure it's completely dry) until frothy. Increase the speed to high, and whisk until the whites form soft peaks. Add the sugar, one tablespoon at a time, and mix at high speed until the whites are glossy and stiff.
3. Spoon the meringue, by the heaping tablespoon, onto the prepared cookie sheets. Space them an inch or two apart. (You should have about 24 meringues.)
4. Bake for 3 hours, or until the meringues are pale and dry and crisp throughout. Turn off the oven and let them dry out in the oven for another hour. Remove by gently peeling meringues off the foil; place in a cool, dry spot. The meringues can be made ahead to this point, and stored in an airtight plastic dish in a cool, dry place until ready to fill.
5. Make the chocolate filling: In a small saucepan, melt the chocolate over very low heat. Add the cream and stir until smooth; the filling will be somewhat thick.
6. To assemble the cookie sandwiches, spoon a heaping tablespoon of sauce on the flat side of a meringue cookie, and press to the flat side of another cookie.
Pre-party prep: Meringues can be made ahead of time (up to a week if you freeze them) and stored in an airtight plastic container, in a cool, dry place, between layers of wax paper to keep them from sticking. Keep them in a cool, dry spot. Assemble the cookies up to 12 hours before serving and bring to party in an airtight plastic container.
How kids can help: Spoon the meringue onto the prepared cookie sheets and peel them away from the foil once baked and cooled; spoon the chocolate sauce onto the cookies and sandwich them together.