Combine 2 cups water, 2 cups granulated sugar and 1 cup brown sugar in a medium saucepan. Heat gently over medium-low heat, stirring constantly until sugars fully dissolve and the mixture comes to a gentle simmer. Do not boil vigorously. Simmer 3–4 minutes to ensure sugars are fully dissolved and the syrup slightly thickens. Remove from heat.

Stir in 2 tablespoons caramel color (or 2 tablespoons prepared caramel syrup) until evenly incorporated to give the syrup its classic dark color.

Whisk in 1 teaspoon citric acid while the syrup is still warm so it dissolves completely. Set the syrup aside to cool to room temperature.

Flavoring (cola concentrate)

Prepare concentrated flavor blend

In a small bowl or jar combine the citrus juices: 1 tablespoon fresh lime juice and 1 tablespoon fresh lemon juice. Add 1 teaspoon vanilla extract and 1 teaspoon phosphoric acid (or the specified food-grade acid solution) and stir to combine.

Add essential oils/extracts one drop at a time: 6 drops orange oil, 4 drops lemon oil, 3 drops lime oil, 1 drop cassia (cinnamon) oil, 1 drop nutmeg oil, 1 drop coriander oil and optionally 1 drop neroli/bitter orange. After each addition, taste a tiny diluted sample (a drop mixed with a teaspoon of syrup) to ensure balance — oils are potent, so add sparingly.

Once the concentrate is balanced, stir the flavor blend into the cooled syrup base until thoroughly incorporated. Taste and adjust acidity (citric or phosphoric acid) and sweetness if needed. If the flavor is too strong, dilute with a little water; if too weak, add more of the oils/extracts in very small increments.

Assembly

Carbonate and serve

Keep the prepared cola syrup chilled. To serve, fill a glass with ice if desired. For standard strength, mix 1 part cola syrup to 5 parts chilled carbonated water (example: 1/2 cup syrup to 2 1/2 cups chilled soda water). For the quantities in this recipe, combine approximately 1 cup syrup with 4–5 cups chilled carbonated water, adjusting to taste.

Gently stir to combine, taking care not to over-agitate the carbonation. Garnish with a lime or lemon wedge if desired and serve immediately.

Store any leftover syrup in an airtight container in the refrigerator for up to 2 weeks. When bottling for longer storage, sterilize bottles and keep refrigerated; do not attempt to carbonate syrup directly in sealed bottles without appropriate equipment.