Heat the oven to 180C/fan 160C/gas 4 and line two baking trays with baking paper. Put the chocolate and butter into a heatproof bowl set over a pan of simmering water and heat, stirring occasionally, until fully melted. Remove from the heat.
Put the sugars and eggs into a large bowl and use an electric mixer to whisk until pale and tripled in volume. Pour the chocolate into the egg mixture and mix to combine. Add all the dry ingredients (except the icing sugar) and mix until evenly combined. Cover the bowl with clingfilm and chill until firm (about 3-4 hours).
To bake, roll the dough into small balls
and roll into a bowl of thr icing sugar, coating liberally. Put the cookies onto the trays, leaving space in between. Bake for 12-13 minutes until puffed and cracked but starting to set around the edges (this is a fudgy, brownie-like cookie so it’s better to err on the side of caution and underbake these). Allow the cookies to cool on the baking trays for 5 minutes before transferring to a wire rack to cool completely. These will happily keep for a week in a sealed container.