For the cake:
Beat egg whites. Set aside. Beat the yolks in a separate bowl with the sugar until thickened. Add in the potato starch and cocoa and mix the yolks again; add in the vanilla. Fold in the beaten egg whites that you set aside from before.
Take out two kitchen towels, run them under water, and squeeze them out as much as possible. Lay them out on the counter in preparation for the cake rolls. Grease and line two jelly roll pans with wax paper, or parchment paper, and grease the paper also. Bake at 375°F / 190°C for 15 minutes. Check with toothpick in middle of cake to see that it is done. Remove the cakes from the oven and immediately flip them out, cake side down, onto the waiting towels. Peel off wax paper carefully, and roll up the cakes in the towel jelly roll style. Let cool for 30 minutes in this position.
For the mousse:
Melt chocolate in double boiler, then add the coffee that was placed in the hot water. Stir together. In a separate bowl, beat egg whites until very stiff. In another separate bowl, beat the yolks until thick, while adding the sugar to the yolks gradually during the beating process. Add chocolate mixture and the vanilla to yolks. Fold both mixtures together. Refrigerate to firm it up.
After the cakes have cooled off completely, carefully open them up and fill them with the mousse filling.
These are the steps for filling the rolls:
Prepare a large sheet of foil, one per roll. Place a large piece of saran wrap on the foil. Unroll the cake rolls directly onto this. Fill, roll up with the help of the saran wrap, and cover gently. Freeze flat, immediately, until hard.
Later you may frost the rolls with the following:
Melt 100 grams hard chocolate together with 2 tablespoons of oil. Spread over rolls, slice when set and serve. You can also grated chocolate bits, ground nuts, coconut, powdered sugar can be sprinkled on top of the frosted rolls according to your taste while the melted chocolate or glaze is still wet.