Place the saucepan over medium heat and warm, stirring frequently, until the sugar is fully dissolved and the mixture comes just to a simmer (do not boil hard). This should take 3–5 minutes.

Add guaraná and flavorings

Reduce heat to low. Whisk in 2 tsp guaraná powder or extract until fully incorporated and no lumps remain. Stir in 1/2 tsp vanilla extract and 1/2 tsp lemon zest.

Simmer briefly

Let the syrup barely simmer on low for 1–2 minutes to bloom the guaraná flavor, stirring so it doesn't scorch. Do not boil vigorously.

Finish and cool

Remove from heat, stir in 1/4 tsp citric acid (or 1/2 tsp fresh lemon juice) to brighten the flavor. Let cool to room temperature, then strain through a fine mesh sieve or cheesecloth into a clean jar to remove any solids. Refrigerate; syrup will keep for 1–2 weeks.

Assemble Guaraná Soda

Chill glassware

If desired, chill two tall glasses in the freezer for 10–15 minutes before serving.

Add ice

Divide 2 cups of ice cubes between the two glasses (about 1 cup ice per glass).

Measure syrup and lime

Into a small measuring cup or mixing glass, combine 1/2 cup guaraná syrup, 2 tablespoons fresh lime juice and 1 tablespoon simple syrup (adjust sweetness to taste). Stir to combine.

Pour soda

Divide the cold carbonated water evenly between the two glasses (about 2 cups per glass or 4 cups total). Pour gently to preserve carbonation.

Combine gently

Pour half of the guaraná mixture into each glass. Stir gently once or twice with a long spoon to incorporate, being careful not to knock out too much fizz.

Taste and adjust

Taste and adjust sweetness or acidity: add more simple syrup for sweetness or a squeeze of lime for acidity. If you used a very concentrated commercial guaraná syrup, reduce quantity accordingly.

Garnish and serve

Garnish each glass with a lime wedge and an optional sprig of fresh mint. Serve immediately with straws.