Preheat oven to 275 degrees. Butter a 10-inch glass pie plate. Beat egg whites in an electric mixer until foamy. Add salt and cream of tartar and beat for one minute at medium speed. When the egg whites start to hold a shape, begin to add the sugar slowly, one spoonful at a time. Add vanilla and beat at high speed for about 6-8 minutes, until very stiff. Use a spatula to spread meringue on bottom and sides of the pie plate. It should be one inch thick and extend about 3/4” above the rim. Bake for 1 1/4-1 1/2 hours until the meringue is very lightly colored. Turn off the oven and let the meringue cool in the oven.
Chocolate Mousse Filling
Place chocolate chips in a large heatproof bowl. Pour hot coffee over chocolate chips and let sit 3-5 minutes. Stir until chocolate is fully melted. Place bowl in microwave for a few seconds if necessary. Set aside until chocolate is cooled. In a chilled bowl with chilled beaters, whip cream with vanilla and Kahlua just until soft peaks form. Fold whipped cream into the chocolate and combine gently. Heap into the cooled meringue shell. Cover loosely with plastic wrap and refrigerate overnight. Before serving, sprinkle with optional chocolate shavings.