Preheat the oven to 170°C/325°F/Gas Mark 3. Put the butter and half the chocolate in a heatproof bowl over a saucepan of simmering water (do not let the bowl touch the water) until the chocolate is melted. Mix the eggs, sugar and vanilla extract in an electric mixer or use a handheld electric whisk. Pour in the chocolate mixture, beating on a slow speed until mixed. Sift the flour, salt and baking powder into a separate bowl, then stir into the chocolate mixture in three stages. Finally, mix in the remaining chocolate. Line two baking trays with greaseproof paper and put six equal amounts of cookie dough spaced evenly apart on each. Bake for 10–15 minutes until the tops look glossy and start to crack. Letthe cookies cool slightly before placingon a wire rack to cool completely.