To make infused gin:
Grind up hibiscus blossoms into a powder. (Try using a food processor or dedicated coffee grinder.) Add powdered hibiscus to the gin bottle and let it infuse for 10 minutes, shaking gently at first. Strain gin through a cheesecloth or coffee filter.
To make syrup:
Combine water and vanilla lavender sugar in a small sauce pot. Bring to a boil while stirring constantly. Lower to a simmer and continue cooking for 3 minutes. Strain and let cool.
To make the cocktail:
Combine infused gin, grapefruit juice, egg white, lavender syrup, and rose water in a shaker with ice. Shake well. Rim the glass with sea salt and strain into a rocks glass with a large ice cube.