Gently melt the chocolate and butter together in a heatproof bowl set over a pan of barely simmering water. Remove and set aside to cool for 10min.
With an electric whisk, combine the eggs and sugar in a large bowl until pale and moussey, about 5min. Fold in the melted chocolate mixture along with the vanilla.
Mix in the flour and bicarbonate of soda. Cover and chill in the fridge for 30min.
Preheat the oven to 180°C (160°C fan) mark 4. Grease and line two baking sheets with baking parchment. Scoop the dough into 10 mounds, spaced well apart on the baking sheets (each cookie will be about 2tbsp). Wet your hands with cold water, then roll the dough into balls, squashing slightly down. Sprinkle with a pinch of sea salt flakes, if you like.
