Preheat oven to 180°C. Combine 250g butter with the caster sugar and lime zest in the bowl of an electric mixer and beat with the paddle attachment on medium speed for 5 minutes, until very light.
Whisk the eggs, yolks and 2 tsp vanilla, then add to the mixer and beat until combined. Fold in the flour and buttermilk and stir to combine. Divide in two, then tint one half red and the other green. Spoon into two lined 20cm x 30cm slice pans and bake for 25 minutes, until just firm. Cool on a wire rack.
Melt the white chocolate and Copha gently, then mix in the cherries, coconut and rice bubbles, then spread over the red cake. Top with the green cake then refrigerate until firm.
Cut the cake into squares. Put the icing sugar, butter and remaining vanilla in a bowl, add ½ cup boiling water, and whisk until smooth. Dip the cake squares in the icing, then roll in coconut.