Spread a thin layer of melted chocolate on to a sheet of baking parchment. Drizzle more melted chocolate in a contrasting colour over the top and feather the 2 chocolates together. Leave until set but not brittle.
Cut into 7 cm (3 inch) squares, then leave until brittle before peeling away the paper.
Layer the chocolate rectangles with spoonfuls of mascarpone and raspberries and sprinkle with some nuts to decorate.