1) Place the sugar in a bowl, cover with boiling water and whisk. Sprinkle the gelatine over the solution, and whisk lightly to incorporate. Let the gelatine mixture cool to room temperature.
2) Pour the gelatine mixture into a large pitcher. Pour out a 250ml glass of the cava and pour the rest of the cava, on an angle, into the pitcher to minimise foam. Stir gently with the whisk. Use the reserved 250ml of cava for another purpose.
3) Pour into six wine glasses and refrigerate immediately. Refrigerate until set, at least 2 hours. Garnish the glasses with a few raspberries.