Let egg whites stand at room temperature for 30 minutes. Meanwhile, line a baking sheet with parchment paper or foil. Draw a 7-inch circle on the paper or foil.
In a medium mixing bowl combine egg whites, cream of tartar, and salt. Beat with an electric mixer on medium speed until soft peaks form (tips curl).
Add the sugar, 1 tablespoon at a time, beating on high speed until very stiff peaks form (tips stand straight) and sugar is almost dissolved (about 4 minutes).
Gently spoon egg white mixture into a pastry bag fitted with a pastry tube. Pipe egg white mixture onto the circle on the paper or foil, building up the side to form a shell. (Or use the back of a spoon to spread the egg white mixture over circle, building up side.)
Bake in a 300 degree F oven for 40 minutes. Turn off oven. Let shell dry in oven, with door closed, for at least 1 hour. Remove from paper or foil; cool completely on a wire rack.
To serve, in a small bowl fold yogurt into sour cream. Spread about two-thirds of the mixture in the shell. Fill with fresh fruit. Pass remaining sour cream mixture. If desired, sprinkle with crystallized ginger or coconut. Makes 6 servings.