Make the strawberry puree: Place the strawberries and powdered sugar in a small blender with the brandy and strawberry liqueur, and purée until totally smooth, sieve the puree through a fine mesh to clear any pips (this can be done in advance and kept in the fridge) – thicken it with a little extra sugar if needed.
Spoon some strawberry puree into a chilled champagne glass then add the champagne, to top it off. Do this slowly, poured down the edge of the glass, do not stir it or let it blend in fully.
Serve garnished with a whole strawberry (with a few of the little green leaves left on standing proud) cut with a slit at its tip and sat on the glass rim, not in the cocktail.
