Combine 2 cups granulated sugar and 1 cup water in a small saucepan. Heat gently over medium-low, stirring continuously until all sugar dissolves and mixture reaches a gentle simmer. Do not boil vigorously. Remove from heat when clear and slightly thickened (about 4–6 minutes).

Add color and body

While the syrup is still warm, stir in 2 tablespoons caramel color (or 2 tablespoons prepared caramelized sugar) and 1 tablespoon brown sugar until fully dissolved. This gives the syrup the characteristic dark color and a hint of molasses depth. Cool syrup to room temperature.

Flavor Concentrate

Combine acidic and citrus notes

In a small bowl, combine 1 teaspoon fresh lemon juice and 1 teaspoon fresh lime juice. Add 2 teaspoons phosphoric acid solution (or the specified diluted vinegar substitute) and stir to combine. This mixture provides the tart, tangy backbone.

Add aromatics and caffeine

To the citrus-acid mixture add 1 teaspoon vanilla extract and 1 teaspoon cola flavor extract (or a blended pinch of orange, cinnamon, and nutmeg essential food-grade oils). If using caffeine powder, carefully weigh 50 mg and dissolve it in a few drops of warm water, then add to the concentrate. Stir until homogenous.

Combine with syrup

Pour the cooled syrup into the flavor concentrate and stir thoroughly until evenly blended. Taste a small amount diluted (1 part syrup to 4–6 parts water) and adjust: add tiny amounts of citrus or vanilla to balance sweetness and acidity. Chill the completed syrup concentrate in the refrigerator if not using immediately.

Bottling & Serving

Dilute with carbonated water

To serve, combine 1 part syrup concentrate with 4 parts cold carbonated water (for this recipe: 1 cup syrup to 4 cups soda water). In a large pitcher or individual glasses, pour desired amount of cold soda water, then add syrup slowly to taste (recommended ratio 1:4). Stir gently to mix—avoid vigorous stirring to preserve carbonation.

Serve

If desired, fill glasses with ice. Pour the carbonated cola over ice and garnish with a thin lemon or lime wheel. Serve immediately. Store leftover syrup in a sealed container in the refrigerator for up to 2 weeks.