Not your typical tonic, this combines lime and lemon juice with cinchona bark, the source of quinine for tonic. Buttermilk peels fresh grapefruits and chars the skins to help bring out bitter yet refreshing notes. Just a bit of Charleston's Bulls Bay Saltworks sea salt helps pull savory caramel notes and rounds out flavor. Works well with any light spirit. Build over ice equal parts mixer plus vodka plus soda water in a tall glass. Garnish with a lime wedge.