Line a baking tray with baking paper and pour the white chocolate into the centre. Use the back of a spoon to shape into a thick rectangle, about A5 size (approximately 21 x 15cm). Drizzle over the dark chocolate and swirl into the white chocolate using a teaspoon handle or similar, then scatter over the coffee beans. Put in the fridge to set. If you want to give the slab as a gift, wrap in cellophane and tie with ribbon. Break into pieces to serve.