01 - Preheat the oven to 350°F (180°C). Grease and line a 8 x 12 inch (20 x 30 cm) baking tin with parchment paper.
02 - Using an electric mixer, cream the softened butter and caster sugar together until pale and fluffy. Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
03 - Alternately fold in the sifted self-raising flour and milk in two batches, mixing until just combined. Be careful not to overwork the batter.
04 - Spread the batter evenly into the prepared baking tin. Bake for 12 to 15 minutes, or until a skewer inserted into the center comes out clean.
05 - Allow the sponge to cool in the tin for 10 minutes, then turn it out onto a wire rack to cool completely.
06 - Meanwhile, dissolve the jelly crystals in boiling water, stirring until fully dissolved. Stir in the cold water, then pour the mixture into a shallow dish. Refrigerate for 45 to 60 minutes until it begins to set but is still liquid enough to coat the cakes.
07 - Once the sponge is completely cool, cut it into 24 equal squares using a sharp knife.
08 - Whip the heavy cream with powdered sugar and vanilla extract until firm peaks form.
09 - Sandwich pairs of sponge squares together with a generous spoonful of whipped cream, creating 12 filled cakes.
10 - Dip each assembled cake into the semi-set jelly, coating all sides evenly. Allow any excess jelly to drip off briefly.
11 - Immediately roll each jelly-coated cake in desiccated coconut, covering all surfaces. Place on a wire rack, then refrigerate for at least 30 minutes to set before serving.