12g powdered gelatine dissolved in 50 millilitres of
1cm thick sponge cake, cut to the same size as
your silicone mould
Make the watermelon shell - Put the white chocolate into a large glass bowl over a saucepan of water. When the water comes to a boil, turn off the heat and stir the chocolate. Use a fine paintbrush to paint white stripes into the silicone mould. Leave to chill in the fridge.
Prepare the green candy melts in the same way, over a saucepan of boiling water. When the water starts to boil, turn off the heat and stir until the candy melts are all melted. Spoon into the silicone mould and spread around until it has a thick coating. Put the mould into the freezer while you make the watermelon jelly.
Make the jelly - Warm the water and dissolve the gelatine powder in it. Add the watermelon juice and mix though, then pour it into a deep tray and put it in the fridge for 30 minutes.
Make the mousse - Whisk the cream to soft peaks then add the icing sugar and lemon zest. Slowly add the dissolved gelatine and then whisk to firm peaks. Put into a piping bag fitted with a 1 centimetre round tip.
Take the mould out of the freezer and pipe mousse in an even layer inside it, leaving a 1 centimetre gap at the top. Use the back of a spoon to smooth it over and then put it into the fridge.
When the jelly has started to set, sprinkle the choc chips over it and stir them through. Fill the watermelon mousse case with the jelly, leaving a 1 centimetre gap at the top.
Position the sponge cake on top of the jelly, neatly inside of the watermelon shell. Cover with cling film and leave for 3 hours to set fully, then turn out onto a serving plate and smash your way into it.