01 - Preheat 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 butter and caster sugar together until pale and fluffy. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
03 - Alternately fold in the sifted self-raising flour and milk in three additions, beginning and ending with the flour. Fold gently until just combined — do not overmix.
04 - Spread the batter evenly into the prepared tin. Bake for 12–15 minutes until the top springs back lightly when touched and a skewer inserted in 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 - Dissolve the jelly crystals in boiling water, stirring until fully dissolved. Stir in the cold water. Pour the mixture into a shallow dish and refrigerate for 45–60 minutes until it reaches a soft, semi-set consistency.
07 - Using a sharp knife, cut the cooled sponge into 24 equal squares.
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 in the center.
10 - Dip each assembled cake into the semi-set jelly, turning to coat all sides evenly. Allow excess jelly to drip off briefly.
11 - Immediately roll each jelly-coated cake in desiccated coconut, covering all surfaces. Place on a wire rack and refrigerate for at least 30 minutes before serving to allow the jelly to set firmly.