Heat the oven to 160C/fan 140C/gas 3. Melt the chocolate and butter in a bowl over a small pan of simmering water or in a microwave until smooth. Cool slightly. In a separate bowl mix the flour, baking powder and ½ tsp salt.
Beat the eggs and sugar with an electric whisk until pale, about 2 minutes. Reduce the speed and add the chocolate mix and vanilla extract. Add the flour mixture until blended together. Cover the bowl and refrigerate for 1½ hours or even up to 2 days.
Put the icing sugar in a small bowl. Shape the dough into 4cm balls and roll in the sugar. Put them on baking sheets lined with baking paper and press down lightly with your hand to flatten. Bake for 12-15 minutes for soft centres and set edges. Let cool on the baking sheets for 5 minutes and then put on a rack to cool.