Put the eggwhites in a bowl of an electric mixer and put the bowl in a second bowl of hot water and let the whites stand, stirring occasionally, for 15 minutes.

Add the cream of tartar and salt and bet the whites until they just hold stiff peaks. While the whites are being beaten, in a saucepan combine the sugar and 1/4 cup of water. Bring the mixture to a boil, stirring to dissolve the sugar, and boil the syrup until it registers 240 deg. On a candy thermometer. Add the sugar syrup to the whites in a stream, slowly, while beating and beat the mixture until it is cool. Add the mascarpone, rum and chocolate and beat until well combined. Divide the cookie or cake crumbs among 4 stemmed glasses. Drizzle each portion with 2 tablespoons of the coffee and top the crumbs with half the mascarpone mixture. Layer the remaining cookie crumbs, coffee and mascarpone mixture in the same manner. Chill the parfaits, covered for a minimum of 2 hours, or overnight.

Garnish: In a metal bowl, set over a pan of barely simmering water, melt the chocolate. Line a baking sheet with foil and spread the chocolate about 1/8" thick on the foil. Chill the chocolate triangles on the baking sheet until it is just set, but not hard, and with a pastry wheel, cut it into triangles. Chill the chocolate triangles on the baking sheet until they have hardened and peel the foil carefully from them. Garnish each parfait with a chocolate triangle.