In a small saucepan, combine the sugar and water. Heat over medium, stirring until the sugar completely dissolves. Bring to a gentle simmer, then remove from heat and let cool.
Brew the Thai tea
Brew Thai tea extra strong according to package directions. Strain and allow it to cool completely.
Prepare the Thai tea base
In a small pitcher or measuring cup, stir together the cooled Thai tea, 3–4 tablespoons simple syrup (adjust to taste), and vanilla extract.
Fill the glasses
Fill two tall glasses with ice.
Assemble the drink
Pour the Thai tea mixture over the ice, filling each glass about halfway.
Add the cream
Slowly pour in the cream or half-and-half to create a layered swirl.
Finish with fizz
Top each glass with sparkling water. Stir gently if you prefer a fully blended drink.
Garnish & serve
Finish with whipped cream or a drizzle of condensed milk if desired. Serve immediately.
Notes
Brew the tea strong: A concentrated brew keeps the tea flavor from getting lost once cream and soda are added.
Cool completely: Hot tea will flatten the soda and melt the ice too quickly.
Sweetness control: Start with 3 tablespoons simple syrup and adjust to taste.
Layered look: Pour the cream slowly over the back of a spoon for a pretty swirl effect.
Stir or sip: Leave layered for presentation or stir fully for an even, creamy drink.
Dairy-free option: Coconut milk or oat cream work especially well with Thai tea flavors.
Make ahead: Simple syrup can be made up to 2 weeks in advance and refrigerated.
Serve immediately: This drink is best fresh while fizzy.