Preheat the oven to 170C/345F/Gas 3½. Line a baking tray with non-stick baking parchment.
Place the butter and chopped dark chocolate in a bowl suspended over a pan of simmering water (do not let the bowl touch the water). When melted, remove from the heat and allow to cool slightly.
Place the eggs, sugar and vanilla in a large mixing bowl and whisk with a handheld electric mixer and until creamy.
Beat in the melted chocolate and butter mixture, then beat in the flour, salt and baking powder.
Stir in the chocolate chips.
Use a tablespoon to place small heaps of the mixture on the lined baking tray at least 5cm/2in apart. Bake each tray for 10-15 minutes until the cookies are still soft in the centre but set around the edge.
Allow the cookies to cool on the baking tray for a few minutes before transferring them to a wire rack to cool completely.