In a small saucepan combine 150 ml water and 200 g granulated sugar. Add orange zest, lemon zest, ground cinnamon, ground nutmeg, crushed coriander, sliced ginger, star anise and split vanilla bean (or reserve vanilla extract).

Bring mixture to a gentle simmer over medium-low heat, stirring until sugar dissolves. Do not boil vigorously; maintain a gentle simmer for 8–10 minutes to extract flavors from the spices and zest.

Remove from heat and let the syrup steep for 30 minutes to 1 hour, covered, to intensify flavor. If using dried lavender add it during the steeping phase to avoid overpowering the syrup.

Strain the syrup through a fine-mesh sieve or cheesecloth into a clean container, pressing solids to extract liquid. Discard solids and let syrup cool to room temperature.

If you did not use a vanilla bean, stir in 5 ml vanilla extract once the syrup is cooled. Add 5 ml caramel color to achieve the dark cola appearance (adjust if desired). If adding caffeine powder, dissolve 50 mg food-grade caffeine into a small amount of warm syrup before combining with the rest. Taste and adjust: you can add up to 25–50 ml more syrup for a sweeter, stronger cola flavor but keep in mind balance with carbonation.

Assembling Coca-Cola (serving)

Chill carbonated water thoroughly (ideally 1–2°C). Use chilled glass or bottle.

Measure 200 ml of the prepared cola-flavored simple syrup and pour it into a pitcher or directly into a serving glass.

Slowly add 1 liter chilled carbonated water to the syrup. Stir gently with a long spoon to combine without losing carbonation. Taste and adjust: add more syrup (10–20 ml at a time) if you prefer a sweeter, stronger cola; alternatively dilute with more carbonated water.

Add 15 ml fresh lime juice and 2 ml vanilla extract to the mixed soda, stir gently to incorporate.

Add 4 ice cubes to glasses, pour the finished cola over ice, garnish with a lemon or lime wheel if desired, and serve immediately.

Store any leftover syrup refrigerated in a sealed container for up to 2 weeks. Do not store mixed carbonated soda — carbonate before serving for best fizz.