Add the orange peel, lemon peel, ground cinnamon, ground nutmeg and dried lavender (if using). Bring to a gentle simmer, reduce heat to low and simmer for 10 minutes to extract flavors. Stir occasionally to prevent sticking.

Remove from heat and let steep for 20–30 minutes, covered, to deepen the flavor.

Strain the mixture through a fine mesh sieve or cheesecloth into a clean bowl, pressing peels to extract liquid. Discard solids.

Stir in vanilla extract and caramel syrup for color. If using food-grade caffeine, dissolve it in a teaspoon of warm water and stir in now. Taste and adjust: add a touch more caramel for color or a pinch more cinnamon for warmth.

Allow the cola syrup concentrate to cool to room temperature, then transfer to a sealed jar. Refrigerate up to 2 weeks.

Simple syrup

Combine 1/2 cup water and 1/2 cup granulated sugar in a small saucepan. Heat over medium, stirring until sugar fully dissolves and the mixture just begins to simmer. Remove from heat and cool. Store in the refrigerator up to 2 weeks.

Assemble the Coke (single serving)

Fill a tall glass with 1 cup ice cubes.

Pour 2 cups chilled carbonated water over the ice.

Add 1 tablespoon simple syrup and 1 teaspoon cola syrup concentrate to the carbonated water. Stir gently once or twice to combine. Do not vigorously stir to preserve carbonation.

Taste and adjust: add up to 1/2 teaspoon more cola concentrate for stronger cola flavor or an additional teaspoon simple syrup for more sweetness.

Garnish with a lime or lemon wedge on the rim if desired. Serve immediately while fizzy.