1Remove paper from mud cakes and place, icing side down, on a baking paper-lined baking tray. Brush tops with the cooled coffee.
2Use electric beaters to beat mascarpone and cream in a bowl until stiff peaks form.
3Place one cake, icing side down, onto a cake board or serving plate. Top with a little of the cream mixture and use a palette knife to smooth the surface. Place remaining cake, icing side up, on top of the cream layer. Place half of the remaining cream mixture in a piping bag fitted with a large plain nozzle and set aside. Cover top and side of cake with remaining cream mixture, smoothing with a palette knife.
4Dip the end of one savoiardi into the melted white chocolate and attach, chocolate-end up, to the side of the cake. Dip the end of another savoiardi into the melted dark chocolate and attach, chocolate-end up, to the side of the cake next to the white chocolate savoiardi. Repeat, alternating white and dark chocolate, with remaining savoiardi until the side of the cake is covered.
5Pipe peaks of the reserved cream mixture over the top of the cake to cover. Dust with cocoa powder to serve.