In a small saucepan, combine 0.5 cup water and 0.75 cup granulated sugar and the 2 tablespoons brown sugar. Warm over medium-low heat, stirring until all sugar dissolves and the mixture is clear (do not boil vigorously). This creates a simple syrup with a touch of brown-sugar/melanoidins to mimic cola depth.

Add flavor components

Remove the syrup from heat. Stir in 1 teaspoon dark molasses until fully incorporated. Add 0.5 teaspoon vanilla extract, 1 teaspoon caramel color, 0.5 teaspoon cola flavoring or cola extract (if using), and 0.25 teaspoon white vinegar. Mix thoroughly. Taste carefully and adjust: more vanilla for roundness, more cola extract for cola character, or an extra 1/4 tsp vinegar for sharper tang. Let cool to room temperature.

Soda Assembly

Combine syrup and citrus

In a pitcher, add the cooled cola flavor concentrate (the entire syrup prepared above). Stir in 1 tablespoon fresh lemon juice and 1/2 tablespoon fresh lime juice. These bright acids are important to imitate the citrus top-notes in commercial cola.

Sweeten and dilute

Add the remaining granulated sugar if the syrup needs extra sweetness (the syrup above used most sugar; if you prefer sweeter soda, dissolve up to an additional 1-2 tablespoons granulated sugar in a splash of warm water and add). This recipe assumes 0.75 cup granulated sugar and 2 Tbs brown sugar total; ensure the syrup tastes pleasantly sweet but not cloying.

Carbonate

Fill a chilled pitcher or individual glasses with 4 cups (about 1 liter) chilled carbonated water. Gently pour the cola syrup mixture into the carbonated water while stirring slowly and briefly to combine. Stir just enough to mix without flattening the carbonation. Taste and adjust: add another 1/8–1/4 tsp vinegar for more bite, or a few drops more cola extract for deeper flavor.

Serve

Pour the finished Pepsi-style soda over ice if desired. Garnish with a lemon or lime wedge. Serve immediately to preserve carbonation.

Notes & Storage

Store any unused cola syrup (concentrate) in an airtight container in the refrigerator for up to 2 weeks. To serve later, mix about 1 part syrup to 8 parts carbonated water (adjust to taste). Fresh citrus juice should be added at serving time for best brightness.