Heat the oven to 160c/fan 140c/gas 3. Melt 175g plain chocolate and 4 tbsp unsalted butter in a bowl over a small pan of simmering water. Cool slightly.
In a separate bowl mix 175g plain flour, ½ tsp baking powder and ½ tsp salt. Beat 2 eggs and 150g golden caster sugar with an electric whisk until pale.
Reduce the speed and add the chocolate mix and 1 tsp vanilla extract. Add the flour mixture until blended.
Cover the bowl and refrigerate for 1½ hours to 2 days.