Melt the chocolate with the butter and chilli over simmering water in a heat proof bowl or in small bursts in the microwave. Once melted, let it cool to room temperature.
Whisk egg yolks with half of the sugar until frothy in a heat proof bowl. Place the bowl over simmering water and whisk until the yolk mix forms pale thick ribbons and doubles in size.
Stir in vanilla. Leave to cool to room temperature.
Whisk egg whites (with a clean dry beater) with a pinch of salt. When it starts to foam add the rest of the sugar and whisk until stiff peaks are formed and glossy.
First fold in the chocolate and egg yolk mix until well mixed through.
Transfer about 1/4th of the egg white into the chocolate mix and fold through to loosen the chocolate.
Fold in the rest of the egg whites gently until just combined.
Pour into 2 serving glasses and let it set in the fridge for at least 3 hours (or overnight) if serving it, as is.