TO MAKE CAKE: Preheat oven to 350°F; grease a 9 x 13-inch pan. Line pan with wax paper and grease the wax paper.
Beat the egg whites lightly, add cream of tartar and beat at a high speed.
When shiny, gradually add 1/3 cup of sugar, continuing to beat until stiff.
In a separate mixing bowl, place the egg yolks, remaining sugar and butter and beat until light and fluffy. Add the cocoa and mix thoroughly. Add a large dollop of egg whites to chocolate mixture and whip until blended.
Then pour the rest of the egg whites over the chocolate mixture and sprinkle the flour over the egg whites. Fold gently with a spatula until the egg whites and flour are incorporated.
Immediately spread the batter in the prepared pan. Smooth out to edges and corners. Bake 10-15 minutes at 350°F until cake is done. Run a knife around edges and invert cake on a wire rack to cool.
TO MAKE FILLING: in a saucepan, combine cream, chocolate chips and instant coffee.
Cook over low heat until mixture is smooth and slightly thickened.
Cover saucepan and refrigerate until very cold.
When chilled, add rum (if using) and whip in a chilled bowl at medium speed until firm. Be careful not to over beat or the filling may separate.
TO MAKE GLAZE (do NOT make this until ALL OTHER steps are completed): in a small saucepan, melt all ingredients over a low flame. Mix until very smooth.
Cool for 15 minutes and mix again. Use immediately, as the sauce thickens as it cools.
TO ASSEMBLE: cut cake horizontally into halves. Lightly sprinkle bottom half with rum. Spread filling over bottom layer.
Place top layer on bottom half and spread filling over top.
Refrigerate or freeze until very firm.
When very cold, set the cake on a wire rack. Pour the warm glaze over the cake. Refrigerate again until firm. Cut with a sharp knife, wiping the blade after each cut.