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Heat oven to 180C/fan 160C/gas 4. Butter and bottom-line 2 x 20cm sandwich tins. For the cake, break the white and dark chocolate into two small bowls, then melt in the microwave on High for 1 min or over a pan of simmering water. Put everything else, except the coffee and vanilla, into a large bowl, then beat until creamy.
Divide the mixture in two, then add the coffee and melted dark chocolate to one bowl, and the vanilla and melted white chocolate to the other. Stir until mixed through, then tip into the tins. Bake for 20-25 mins until risen and slightly shrunk from the sides of the tins. Cool in the tins for a few mins, then turn onto a wire rack.
For the ganache, heat the cream in a pan, then break the dark chocolate into a large bowl. Once the cream is just boiling, pour it onto the chocolate, then leave for 5 mins. Stir until smooth, then leave until thickened and cool. Split the cakes in two across the middle (a bread knife works well), layer up the pieces, alternating white and dark layers, and sandwiching, then topping them with the ganache.
To decorate, spoon the melted white chocolate into a piping bag with a fine nozzle (or use a sandwich bag and snip off the end instead). Zig-zag the white chocolate over the cake, then finish with a scattering of white chocolate malt balls. Serve on its own or for dessert with pouring cream. Best eaten on the day, but keeps in an airtight container for up to 3 days.