In a large mixing bowl whip 150 ml cold heavy cream until very thick (before starting, allow it to sit for 6-8 hours on the top each of the fridge).
In a medium heatproof bowl combine the chocolate, broken into pieces, the instant coffee and 50 ml 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.