Put the 250 g Guittard 70% Bittersweet Dark Chocolate Baking Bar, butter and coffee in a heatproof bowl set over a pan of gently simmering water, making sure the base of the bowl doesn’t touch the water. Leave the chocolate and butter to melt, stirring occasionally.
Once you have a smooth mixture, remove the bowl from the heat. In a large separate bowl, whisk the egg yolks and the caster sugar together for 5 minutes or until the yolks are pale and thick. Add the alcohol, if using, and whisk to combine.
Put the egg whites in a clean, grease-free bowl and, using an electric mixer, whisk until they hold stiff peaks.
Add the chocolate mixture to the egg yolks and mix to combine. Working in three additions, carefully fold in the egg whites using a spatula, folding until you can no longer see any streaks of white.
Pour the mousse into a large serving bowl or divide among 6-8 serving glasses. Chill in the fridge for at least 4 hours to set. Serve topped with a little lightly whipped cream and some crushed amaretti biscuits if you wish.
Notes
This dessert will keep for up to four days stored in the fridge.