Mix blucha powder with a little bit of warm water so that it dissolves completely. Then top it with more water.
Then stir in the lavender, lemon zest.
Place the drink mixture in the fridge to chill for at least an hour.
Rub the lemon slice around the rim of the cocktail glass.
Dip the rims of the glass on a plate of the spread out caster sugar until the sugar is well coated.
Let the glass stand for a few minutes for the sugar to set, before pouring the mocktail into the glass.