Add everything except jelly and beat for a few minutes. Pour into a cake tin and bake at 180 deg cel until skewer comes out clean. Make up half the jelly box using 1/2 cup boiling water and 100mL soda water and pop in fridge while cake cooks. Once out of oven let cool for 10 or so minutes. Attack the top of cake with a skewer then spoon the jelly on top and spread around evenly as it soaks in. Let cool fully. If you want to ice, beat 60g soft butter with remaining jelly crystals and 1tbsp soda water. Add icing sugar until the right consistency and smear away.