Preheat oven to 350 degrees Fahrenheit.

Mix coffee, hot water and two tablespoons sugar in a small bowl until dissolved. Let cool.

Beat egg whites with an electric mixer until soft peaks form. While mixer is running on medium-high speed, slowly add remaining cup of sugar until stiff peaks form. Turn mixer to lowest setting and add egg yolks, oil, potato starch, vanilla sugar, and cooled coffee. Mix just until incorporated.

Pour into a Gefen Parchment Paper-lined baking tin and bake for 45 to 55 minutes, until puffed and golden and a toothpick comes out clean.

For icing, dissolve coffee granules in hot water, then mix with powdered sugar. Once cake is completely cooled, top with icing.