In a small saucepan combine 0.5 cups cold water, cane sugar (or chosen sweetener) and caramel color (or 1 tbsp dark brown sugar). Heat over low, stirring until sugar fully dissolves and the mixture is slightly thickened, about 3–4 minutes. Do not boil. Remove from heat.

Add spices and flavorings

While the syrup is warm, whisk in vanilla extract, lime juice, ground cinnamon, nutmeg, and coriander. Add the citrus oil drops one at a time, tasting after each drop to avoid overpowering. Stir until fully incorporated.

Finish syrup

Stir in citric acid and (if using) caffeine powder until dissolved. Allow the syrup to cool to room temperature, then add the remaining 1.5 cups cold water and mix thoroughly. Strain the syrup through a fine mesh sieve or coffee filter to remove any undissolved spice particulates. You should have approximately 2 cups of cola syrup concentrate.

Assemble Drink

Chill a tall glass. Fill the glass with about 1 cup of ice cubes.

Measure 1/4 cup (60 ml) of the prepared cola syrup concentrate and pour it over the ice.

Slowly add 3/4 cup (180 ml) very cold carbonated water to the syrup (ratio about 1:3 syrup to carbonated water). Gently stir once to combine without flattening the carbonation. Taste and adjust: add more carbonated water for a lighter flavor or more syrup for stronger taste.

Garnish with a lime wedge, squeeze if desired, and serve immediately.