For the lavender ice cubes: Place a sprig of fresh lavender in 24 large square ice cube molds. Fill the molds with water and freeze until solid, 2 hours.
For the punch: Combine 1 1/4 cups water and the hibiscus and dried lavender in a small saucepot. Bring to a simmer, then turn off the heat and let steep for 30 minutes. Strain and discard the lavender. Cool in the refrigerator.
To a large pitcher, add the hibiscus-lavender water, lemon juice, seltzer, vodka, all the Honey Simple Syrup and a few lavender ice cubes. Stir to combine and top off with the prosecco.
Serve the punch in glasses over lavender ice cubes.
Honey Simple Syrup:
Put the honey and 1/2 cup water in a small saucepot over medium-high heat. Bring to a simmer, stirring to dissolve the honey. Remove from the heat and let cool completely.