Preheat the oven to 170C/Gas 3/fan oven 150C. Grease a 23cm/9in springform cake tin and line the base with greaseproof paper or baking parchment. Dust the sides with a little flour. Put the chocolate and butter into a heatproof bowl set over a pan of gently simmering water. Heat until melted, then remove the bowl from the pan and stir until smooth. Leave for about 5 minutes to cool slightly.
Stir in the egg yolks, ground almonds, and the liqueur, if using. Put the egg whites into a large bowl, add a pinch of salt and whisk until soft peaks form. Continue whisking, sprinkling in the sugar a little at a time, until stiff peaks form. Stir 2 tablespoons of the whites into the chocolate mixture, then carefully fold in the remainder until no traces of white are left.
Spoon the mixture into the prepared tin and bake for 30-35 minutes until well risen and just firm to the touch. Cool in the tin (don’t worry if the cake sinks and cracks slightly – it will still be fine). You can make the cake up to this stage up to four days ahead and keep it in an airtight container in a cool place. It also freezes well for up to one month
To finish make the ganache: Chop the chocolate into small pieces and slide into a mixing bowl. Pour the double cream into a pan, add the caster sugar and 1 teaspoon of the sea salt heat until it is almost at boiling point. Take off the heat and pour over the chocolate. Stir until the chocolate has melted and the mixture is smooth. Leave to cool, stirring occasionally.
Pop the cake on a serving plate or cake stand. Pour the ganache onto the centre of the cake and ease outward and down the sides with a palette knife- don’t be in a hurry to do this, it is easier when the ganache is cool enough to hold it’s shape rather than run too freely. Finish with a sprinkle of the remaining vanilla sea salt. Slice and serve with coffee- cream.
Coffee Cream- 300ml double cream + 1 teaspoon vanilla extract + 1 tablespoon icing sugar (sifted) + 4 tablespoons espresso coffee. Whip together until soft creamy peaks form.