Preheat the oven to 180C/350F degrees.
Grease and line an 8 inch spring form cake pan with parchment paper.
Melt the butter and chocolate together in a bowl over simmering water (Bain Marie). Or you could zap it in the microwave.
In a bowl, whisk the egg yolks together with 2 tablespoons of the erythritol and the vanilla.
Add the chocolate mixture, then add the hazelnut flour, salt and 1/4 cup of the erythritol. Stir well.
In another bowl, whisk the egg whites to soft peaks with an electric whisk.
Add the remaining erythritol and whisk until you have stiff peaks.
Add 1/4 of the egg white mixture to the chocolate mixture and stir slowly.
Fold the remaining egg whites into the mixture.
Pour into the cake tin and bake for 35 to 40 minutes.
Eat and enjoy!