Combine 1/2 cup water with the sugar, black salt and mint in a small saucepan. Heat over medium-high heat until the sugar and salt dissolve into a sweet and salty simple syrup. Strain the syrup and transfer to the fridge to cool. Discard the mint leaves.
Squeeze the limes into a pitcher and add the chilled simple syrup.
Crush or muddle a few mint leaves in the bottom of an 8-ounce mint julep cup until they form a paste. Fill halfway with ice. Add 2 ounces of the simple syrup and the bourbon and top with seltzer. Stir until the silver cup is frosted on the outside.
To serve, garnish with a sprig of fresh mint. Reserve the remaining simple syrup for additional drinks.
A glass tumbler will not frost on the outside.
Recipe courtesy Maneet Chauhan, "Flavors of My World" (Southwestern Publishing Group)