Preheat oven to 180 degrees C (350 degrees F). Line a large baking pan (approximately 10 x 13 inch / 25 x 35 cm) with a sheet of parchment paper.
In a small bowl combine flour and cornstarch and mix to blend.
In a large bowl beat together egg yolks, powdered sugar and 1 teaspoon vanilla extract until creamy. Add flour and cornstarch and mix carefully with a rubber spatula until combined.
In another large bowl whip the egg whites and salt until very thick. Carefully fold in the whites to the yolk mixture until well combined.
Pour the batter into the prepared pan and bake for 15-20 minutes or until golden.
Sprinkle some powdered sugar over a clean tea towel. Place the baked cake on the towel (with the paper on top). Remove the paper and trim any uneven edges of the cake. Carefully roll up the the cake together with the tea towel. Let cool for 30 minutes.
In a large mixing bowl whip 2/3 cup (150 ml) cold heavy cream until very thick.
In a medium heatproof bowl combine the chocolate, broken into pieces, the instant coffee and the remaining heavy cream. Place the bowl over a pan of gently simmering water. Stir until the chocolate melts.
With a rubber spatula carefully fold the chocolate mixture into the whipped cream until homogeneous. Let cool in the fridge for 1-2 hours before assembling the roll.
Unroll the cake and remove the towel. Place the cake on top of a sheet of parchment paper. Sprinkle with 1 teaspoon vanilla extract dissolved in 2 tbs water. Distribute the chocolate mousse evenly on top and roll the cake carefully, using the paper as a support. Wrap the rolled cake into the paper and let stay in the fridge for 1-2 hours.
Before serving sprinkle the roll with powdered sugar.
Cool the heavy cream for at least 6-8 hours at the top rack of the fridge, before whipping it.
The same roll can be filled also with jam, buttercream or another kind of frosting, if you prefer.
Keep the leftovers in airtight container in the fridge for 2-3 days.