In a small saucepan combine 1 cup water, brown sugar, granulated sugar, molasses, sarsaparilla root, birch bark, chicory root, dried licorice root, sliced fresh ginger, whole allspice berries, whole cloves and ground cinnamon. Stir briefly to combine.

Bring the mixture to a gentle simmer over medium heat. Once simmering, reduce heat to low and let it gently simmer for 10 minutes, stirring occasionally so sugars dissolve and the flavors infuse. Do not boil vigorously — keep it at a low simmer to concentrate flavor without burning sugars.

After 10 minutes, remove the saucepan from heat. Add the vanilla extract, molasses (if not already added), and citric acid (or lemon juice). Stir to combine.

Allow the syrup to cool for 15–20 minutes. Strain the syrup through a fine mesh sieve or cheesecloth into a heatproof container to remove solids (roots, bark, spices). Press on solids gently to extract as much liquid and flavor as possible, then discard the solids.

Chill and carbonate

Refrigerate the strained syrup until thoroughly chilled (about 30–60 minutes) or use cooled syrup. For each 1 cup of chilled carbonated water, use 2–3 tablespoons of root beer syrup, depending on desired sweetness and intensity. For this recipe: add 6 cups chilled carbonated water to a pitcher and stir in 6 tablespoons (3/8 cup) of the syrup, tasting and adjusting to your preference.

If using citric acid, taste the mixed root beer and adjust acidity with up to an additional 1/4 tsp citric acid or a squeeze of lemon if you prefer brighter flavor. If too sweet, add a little more chilled carbonated water; if too weak, add more syrup (1 tablespoon at a time).

Serve immediately over ice if desired and garnish with a maraschino cherry or lemon slice. Store any leftover syrup refrigerated in a sealed container for up to 2 weeks.