To make the herb-simple syrup for the martini, combine the water, sugar and lemon zest in a pot over medium heat. Gently simmer for 3 minutes until the sugar is dissolved, swirling the pot over the heat now and then. Do not allow to boil or get dark. Put the thyme in a mason jar or glass container and pour the sugar syrup on top; allow to steep while it cools for 1-2 hours to infuse the herb flavor into the syrup. Strain to remove the solids.
In a martini shaker filled with ice, combine the vodka, Lemoncello, and 4 tablespoons of the herb-syrup. Shake well until frothy and strain into 4 chilled martini glasses. Float candied lemon rind in each glass for great presentation.