By Fiona Cairns
Buttermilk helps to lighten the texture and adds flavour.
Preheat the oven to 180°C/fan 170°C/350°F/gas mark 4. Butter 2 × 20cm round sandwich tins, each 4.5cm deep, and line the bases with baking parchment.
Place the chocolate in a bowl and pour over 120ml just-boiled water. Stir until melted, then set aside to cool.</