Chocolate truffle cookies

Preheat the oven to 180°C/350°F/Gas mark 4. Combine the broken chocolate pieces, the cocoa powder and butter in a metal bowl and place over a pan of simmering water. Stir occasionally until the chocolate and butter have melted. Remove from the heat, stir to combine and set aside to cool. Beat the eggs and sugar, using an electric mixer, until light and fluffy. Beat in the vanilla extract and chocolate mixture. Sift together the flour, baking powder and salt. Add to the butter mixture together with the chopped chocolate and stir until just combined. Cover the dough and chill for at least 3 hours. Even after chilling, the dough will be quite soft. Quickly roll the dough into heaped teaspoon-sized balls (roughly 3cm/1in wide) and place 5cm (2in) apart on ungreased or parchment-lined baking sheets. Bake for 10 minutes, or until the tops are crispy and the centres just set. Allow to cool for 5 minutes on the baking sheets then transfer to a wire rack and allow to cool completely. Dust with cocoa powder to serve.