Preheat the oven to 180°C. Line two large baking trays with baking paper.
Melt the Las Trincheras chocolate in a heatproof bowl, set over a pan of gently simmering water, making sure the bottom of the bowl is not in contact with the water.
Beat the butter and sugar together until light and ﬂuﬀy. Blend in the beaten egg, and melted chocolate until well combined.
Sift the ﬂour, cocoa powder and bicarbonate of soda together into a large bowl, then mix into the butter, sugar and chocolate mixture.
Stir in the small chunks of chocolate until evenly mixed through.
Spoon heaped teaspoons of the mixture onto the prepared baking trays, spaced 6–8cam apart.
Bake in the preheated oven for 10–12 minutes, or until golden brown. Allow to cool slightly on the trays, then transfer to a wire rack and leave to cool completely.