Place the chocolate and butter in a heatproof mixing bowl over, but not touching a pan of simmering water. Occasionally stir until just melted. [7 minutes]
In a separate bowl, mix together all the dry ingredients except the sugar, and set aside.
Using a stand mixer [Note3], whisk the egg, then slowly add the sugar and whisk at a medium-high speed for 10 minutes, until pale and trebled in size. Then slowly fold in the dry ingredients a spoonful at a time. Next fold in the chocolate mixture.
Place in the fridge to set for 1 hour.
Preheat the oven to 180C fan/gas mark 6. [355F]
The mixture should be quite firm now, so spoon it out into 30g balls. Evenly space them on a baking tray lined with baking parchment.
Bake for 14 minutes until the the biscuits are nicely domed and cracks are showing. Cool on the tray before eating.
Notes
Note 1. Whizz sugar in your food processor until the grains are fine- voila! Caster sugarNote 2. I used a large eggNote 3. I used a hand mixer and a clean, deep mixing bowl