Caramelize the sugar. Add cane sugar to a medium non-reactive saucepan with 1/4 cup water, stirring to combine. Cook over medium high heat until amber and caramelized.*

Flavor the syrup. Carefully add 2 cups of room temperature water and lemon juice. The mixture will bubble and splatter, so stand back while pouring. Once the mixture calms, stir until smooth. Add split vanilla bean and seeds. Bring mixture to a boil then turn off the heat and let steep for 1 hour.

Strain and serve. Pour syrup through a fine mesh basket strainer into a clean bowl. Discard solids. To serve, pour 1 oz. of syrup into small glass filled with ice. Top off with seltzer water and gently stir. If you find the soda to be too sweet, add more seltzer water (or less syrup next time!). If the soda isn’t sweet enough, add more syrup.