Combine butter and sugar in small bowl, beat with electric mixer until light and fluffy, add cooled dark chocolate and eggs, beat until combined. Transfer mixture to large bowl, stir in sifted cocoa and flours in 2 batches, then milk and white chocolate and nuts. Drop heaped teaspoonsfuls of mixture about 5 cm apart onto lightly greased oven trays. Bake in moderate oven for about 10 minutes or until cookies feel firm but slightly soft in the center. Lift onto wire racks to cool. Drizzle or pipe with extra dark and white chocolate. Makes about 36.