First, make the ganache. Whisk the coffee and melted chocolate together until you have a smooth, glossy mixture. Leave to cool and thicken, stirring occasionally.
Heat the oven to 180C/160C fan/gas 4. Line a 23 x 32cm baking tray with baking parchment. For the meringue, beat the egg whites and a pinch of salt with an electric whisk until stiffened slightly.
Add 1 tbsp of the caster sugar to the egg whites, then whisk again to stiff peaks. Repeat with the rest of the sugar, a spoonful at a time, until the mixture is thick and shiny. Spread evenly into the tray, then bake for 15-20 mins, or until crisp to the touch and lightly golden in places. Leave to cool completely.
For the mascarpone cream filling, put the cream, sugar, mascarpone and liqueur, if using, in a medium bowl and whisk until thickened.
To assemble, sift some icing sugar over a large sheet of baking parchment, then carefully flip the cooled meringue onto it. Lift off the tin and peel away the baking parchment. With a short end facing you, score a line 2cm into the edge of the meringue. Spread the ganache evenly over the meringue, then top with the cream, smoothing it with a palette knife, then carefully roll into a roulade, starting from the scored end and using the parchment to help. Transfer to a serving plate and dust with cocoa powder.
