Preheat oven to 160 C / Gas 2/3. Line a baking tray with baking parchment.
Melt the chocolate in a double boiler or over a pan of barely simmering water. Remove from the heat and add the butter. Mix well until the butter disappears and the mixture is smooth and glossy.
In a mixing bowl, beat together the eggs and caster sugar until the mixture becomes pale and fluffy. Add the vanilla extract and pour in the melted chocolate, give a good stir.
In a separate mixing bowl, combine the flour, cocoa powder, salt, and baking powder. Gradually fold this mixture into the melted chocolate mixture and work it until it all comes together. Cover with cling film and chill in the fridge for about 1 hour.
Remove from the fridge and form into balls about 2 to 2.5cm in diameter. Place some icing sugar into a saucer and roll the balls in it before arranging the balls on the prepared baking tray, leaving plenty of room to spread.
Bake for 10 to 12 minutes until firm and with pleasing cracks on the surface of the biscuit. (don’t open the oven door while the cookies are baking). Remove from the oven and allow to cool for at least 10 minutes on the tray before transferring to a cooling rack to cool completely. Once cooled serve and enjoy.