Combine eschalots, champagne and vinegar in small saucepan and simmer for 5 minutes or until reduced to approximately 2 tablespoons of liquid.
Strain, reserving liquid.
Return liquid to pan over low heat and add one cube of butter at a time whisking until all butter has been added. Add lemon for taste. Keep warm.
Brush butterflied prawns with olive oil, season with salt and place under hot grill for 3-4 minutes or until cooked through.
Serve with beurre blanc, watercress and micro parsley.