In a mixer fitted with a whisk attachment (or using a hand mixer), whip the egg whites until foamy. Add the cream of tartar and salt and continue whipping until soft peaks form. With the mixer running, gradually add 1 cup of the sugar and continue whipping until stiff and the sugar has dissolved, about 30 seconds.
In a small bowl, stir together the hot water and instant coffee to make coffee extract.
Using a sieve over a large bowl, sift the remaining 1/2 cup sugar with the pre-sifted cake flour 3 times, to aerate the mixture.
Fold the flour mixture into the egg whites in batches; then fold in the vanilla and dissolved coffee.
Spoon the batter into an ungreased 9 or 10-inch tube pan and smooth the top with a rubber spatula. Bake until light golden brown, about 30 to 35 minutes.
Cool by turning the cake (in the pan) upside down, propped on top of a bottle until it cools to room temperature. Run a long, sharp knife blade, or a butter knife, around the cake to loosen, then knock the cake out onto a plate or work it out with your fingers. The outside crumb of the cake will remain in the pan; exposing the white cake underneath.
Make the Glaze: Stir the ingredients together until smooth. Add more liquid if too thick or more sugar if too thin. Pour glaze over the top of the cake, letting it drip down naturally. Let glaze set until hard, at least 30 minutes before serving. Serve with vanilla frozen yogurt, so you have a cappuccino in your mouth.