Rim: Mix ½ tsp powdered sugar with ½ tsp ground Sencha or Matcha powder on a saucer. Wet the rim of a cocktail glass with lime juice, and gently dip the rim in the sugar mixture.
Shake the first 4 ingredients in a cocktail shaker full of crushed ice, and strain into the rimmed cocktail glass.
Top up glass with chilled champagne or sparkling wine, and garnish with a slice of grapefruit