Preheat the oven to 180 C, 160 Fan, Gas 4. Butter a 23 cm round cake tin and line the base with baking parchment.
Combine the chocolate and salt in a heatproof bowl and set over a pan of simmering water. When the chocolate has melted turn off the heat and drop the butter pieces into the bowl but do not stir.
Let the mixture sit for a few minutes until the butter begins to melt, then stir through briefly and leave for a few minutes more.
In a separate bowl whisk the egg whites to soft peaks, Add the sugar and whisk until stiff peaks form.
Now stir the chocolate mixture until the butter has fully melted. Use a whisk to stir in the egg yolks one at a time, then gently fold in the egg whites,
Fold in the ground almonds being careful not to overmix and knock the air out of the mixture.
Pour the mixture into the prepared tin and bake for 25 minutes or until an inserted skewer comes out clean. Remove from the oven and allow to cool a little before turning out.