Make a syrup with the water and 325g / 1 2/3 cups of the caster sugar by bringing them to the boil in a saucepan and boiling for 5 minutes. Take the pan off the heat, add the lemongrass and let it infuse for about half an hour.
Strain the syrup into a measuring jug, keeping the saucepan with the lemongrass to one side. Take out about 150-200ml / 2/3 – 1 cup of the syrup and put it into a pan with the raspberries. Bring it to a rolling boil and let it thicken slightly, mashing the fruit to make a jam-like consistency.
Let it cool a little and then dunk the trifle sponges in the raspberry mixture and arrange them in the bottom of your bowl. Add the vodka and about 100ml / 7 tablespoons of the lemongrass syrup depending on how much your sponges absorb, and reserve the rest.
Meanwhile, to make the custard, heat the 600ml / 2½ cups single cream in the syrup pan with the lemongrass until it is nearly boiling, take it off the heat and let it infuse for about 15 minutes.
Whisk the yolks and the rest of the sugar together and pour the cream into the same bowl. Then whisk again and put the custard back on the heat in the cleaned-out pan. Stir until the custard thickens and then pour it over the trifle sponges. Let it cool.
Whip the 500ml / 2 cups double cream until thick but not stiff, and cover the custard layer. Use about 250ml / 1 cup of the remaining sugar syrup to make a caramel by heating it in a saucepan until it turns a golden-brown.
Drizzle the caramelised sugar syrup over the layer of cream to decorate.