Place mint, simple syrup and lime juice in a highball glass.
Using a wooden muddler or back of a wooden spoon, muddle mint, syrup and juice together to form a good syrup.
Fill cup with ice cubes.
Pour in vodka and St. Germain. Mix with a spoon.
Top off with club soda and mix one more time.
Garnish with a sprig of mint.
Notes
* SIMPLE SYRUP 1 cup (8 ounces) water 1 cup granulated sugar Bring the water to a simmer in a saucepan set over medium-high heat. Add the sugar and stir until it completely dissolves. Remove the pan from the heat. Set aside to cool to room temperature. Pour the syrup into a clean 1-pint bottle, cap it and refrigerate it until needed. Makes 1½ cups and keeps two months or longer in the refrigerator.