Combine 2 cups granulated sugar, 1 cup water, and 1 cup dark brown sugar in a small saucepan. Heat gently over medium, stirring constantly until sugar is fully dissolved and mixture is just below a simmer (do not boil hard). This should take 4–6 minutes.

Once dissolved, lower the heat and simmer gently for 2 minutes to meld flavors. Remove from heat and allow syrup to cool for 10 minutes.

Flavoring

While the syrup is still warm (but not piping hot), stir in 3 tablespoons cola flavoring concentrate (or an equivalent cola syrup), 1 teaspoon vanilla extract, 1 teaspoon orange zest, 1/2 teaspoon lemon zest, and 1/4 teaspoon ground cinnamon. Stir until completely incorporated.

Taste the concentrated syrup: it should be noticeably cola-like but quite sweet and concentrated. If using a commercial cola syrup, follow its intensity guidance (you may need slightly less or more concentrate). Let the flavored syrup cool to room temperature, then refrigerate until fully chilled (about 30–45 minutes) if time allows.

Acidity & Fizz

Once the flavored syrup is chilled, add 2 tablespoons fresh lemon juice and 1/2 teaspoon phosphoric acid solution or citric acid solution (optional) to the syrup and stir well. This brightens and balances the sweetness.

Transfer the chilled syrup to a clean 2-liter bottle or a large pitcher. Pour in 1.9 liters of very cold carbonated water (soda water or club soda) slowly to preserve as much carbonation as possible. If using a 2-liter plastic soda bottle, add syrup first, then top to the 2 L mark with the carbonated water, leaving minimal headspace.

Gently invert the bottle or stir slowly with a long spoon to incorporate syrup and carbonated water. Do not shake vigorously — you will lose carbonation.

Optional Finishing

Chill the finished 2-litre pop in the refrigerator for at least 30 minutes for flavors to marry. Serve over ice if desired and garnish with lemon slices.

Store tightly sealed in the refrigerator and consume within 3–5 days. Re-carbonate with a soda siphon or fresh club soda if fizz diminishes.