Grate the rind of 1 grapefruit finely, making sure you don't take any of the bitter white pith. Cut the skin and the white membrane off both grapefruits, and cut between the membranes to remove the segments. Place in a large dish, sprinkle with 2 tablespoons of the sugar and set aside.
Whisk the cream in a large bowl until thick but not stiff. Fold in the yogurt, elderflower cordial, spices, grapefruit rind and remaining sugar until smooth.
Spoon the mixture into attractive glasses, arranging the grapefruit segments between layers of grapefruit cream. Sprinkle the top with a little extra sugar, add the brandy snaps, if desired, and serve immediately.