Bring 1 cup water to just below boiling. Add 1 tablespoon (approx.) oolong tea leaves (to make a strong brew) and steep for 4 minutes. Strain and set aside to cool to room temperature (about 10–15 minutes).

Make pandan-oolong simple syrup

In a small saucepan combine 0.5 cup water and 0.5 cup granulated sugar. Add 2 pandan leaves (knotted) or 1 tsp pandan paste. Warm over medium heat, stirring until sugar dissolves. Pour in 1/2 cup of the cooled strong oolong tea. Simmer gently 2–3 minutes to infuse, then remove from heat. Let the pandan leaves steep in the syrup as it cools for 10–15 minutes for stronger aroma. Strain out leaves and any tea solids into a clean jar. Chill. This yields about 1 cup of pandan-oolong syrup; measure 0.5 oz per cocktail.

Cocktail

Prepare mango purée

If not using pre-made purée, peel and dice a ripe mango. Place mango pieces in a blender and puree until smooth. If needed, add 1–2 teaspoons water or coconut water to help blend. Strain briefly for a silkier texture (optional). Measure 2 oz purée per cocktail.

Build the highball

Fill a highball glass with ice cubes to the top. In a shaker without ice (or a mixing glass), combine 2 oz white rum, 2 oz mango purée, 2 oz coconut water, 0.5 oz fresh lime juice and 0.5 oz pandan-oolong syrup. Whisk or shake briefly (5–7 seconds) to combine and lightly aerate—this helps integrate the mango.

Strain and top

Strain the mixture into the prepared highball glass over the ice. Gently pour 3 oz chilled club soda down the side of the glass to top and lighten the texture. Stir once with a bar spoon 2–3 gentle turns to incorporate the soda without fully collapsing the effervescence.

Dark rum float (optional)

Carefully float 0.5 oz dark or aged rum on top by pouring it over the back of a spoon so it rests as a fragrant layer.

Garnish and serve

Garnish with a mango slice on the rim and a pandan leaf or lime wheel. Serve with a straw and advise the drinker to give it a gentle stir before sipping to combine the float.