Boil sugar and water in a saucepan. Mix 2 cups (400g) granulated white sugar into 1 cup (240mL) water. Bring the solution to a boil, stirring until the sugar dissolves.

Mix in vanilla. For the best flavor, scrape out a whole vanilla bean, mix in the goo, then drop the empty pod in as well. You can substitute 1 tbsp (15mL) vanilla extract instead, or use vanilla paste as directed on the label.

Add lemon juice or cream of tartar. Stir in ⅛ tsp (0.6mL) lemon juice or cream of tartar. This will invert the syrup, which makes it sweeter, helps the sugar dissolve, and increases shelf life.

Simmer for 5–10 minutes. Simmer over medium-high heat, reducing heat if bubbles are building up above the surface. The mixture is ready when a candy thermometer reads 220ºF (104ºC), or when it's syrupy, brown, and covered in small, thick bubbles.

Let cool 5 minutes. Move the pan to another burner and wait for it to cool to a safer temperature.

Steep at room temperature for one hour. This gives the syrup time to absorb the vanilla, which is the main flavor of cream soda.

Mix the syrup with carbonated water and ice. When you're ready to drink your soda, stir the syrup into carbonated water (seltzer) and ice. Try 1–2 tbsp syrup per 12 oz glass (15–30mL per 360mL), then experiment to make it as sweet or mild as you like your soda. That's one of the benefits of making it at home!

Add cream or ice cream (optional). Most of the time, cream soda doesn't actually contain cream, but a spoonful of cream or half-and-half does give it a nice foam. For the full dessert option, use a spoonful of vanilla ice cream instead.

Store extra syrup in the refrigerator. It will usually last at least a week.