Preheat the oven to 350° Fahrenheit. Prep a cookie sheet or jelly roll pan: grease with butter or spray, line with parchment paper, then grease the paper. In a small sauce pan, melt the chocolate with the water, stirring constantly. Once melted, remove from heat and let cool. Using an electric mixer with clean beaters, beat egg whites until soft peaks form. Add 1/3 of the sugar and continue beating until stiff peaks form, set aside. In the egg yolk bowl, add the other 1/3 cup of sugar and beat until thicker and lighter, about a minute. Add the starch and salt and beat; while still beating, pour in the cooled melted chocolate until fully combined. Fold in the egg whites 1/3 at a time until smooth and combined. Pour into prepared pan and spread evenly with a spatula. Bake for 15-18 minutes until spongy to the touch, then remove and let cool slightly.

When cake has slightly cooled, lift parchment paper and cake out and place on a dampened kitchen towel. Sprinkle with powdered sugar and carefully roll entire cake using parchment paper and towel. Let sit, rolled, until completely cooled.

While cake is rolled and cooling, make the filling! Mix the instant coffee granules with the cream and stir until fully dissolved. Using the electric mixer, beat the coffee cream and cream of tartar until soft peaks form. Add the sugar and vanilla extract and continue to beat until stiff peaks form. When cake is cooled, carefully unroll. Use a spatula to spread the coffee whipped cream across the entire cake evenly. Re-roll the cake and wrap in the parchment paper - chill in fridge or freezer for 30 minutes to an hour.

While the cake is chilling, make the fudge sauce. Dissolve the instant coffee granules into the milk and set aside. In a large frying pan, melt the butter over medium low heat. Add in cocoa powder and whisk until smooth, then add in sugar and whisk until combined. Add milk and continue to whisk until thick. Remove from heat and let sit until slightly cooled.

Remove cake from freezer or fridge and unwrap. Plate the cake and pour the fudge sauce over the cake and top with chocolate curls if you want! Cake can keep in the fridge or freezer.