Place the butterfly pea flowers in a heat-safe mug or measuring cup. Pour boiling water over the flowers and steep for 5 minutes until the liquid turns deep sapphire blue.
Strain out the flowers using a fine-mesh strainer or tea infuser, discarding the solids. Stir in your preferred sweetener while the tea is still warm, mixing until completely dissolved.
Let the sweetened tea cool for 2-3 minutes, or place it in the refrigerator for faster cooling. You want it at least room temperature before assembling.
Fill a tall glass with ice cubes. If desired, froth your cold milk with a handheld frother for 10-15 seconds to add creamy texture.
Pour the cold milk slowly over the ice, filling the glass about two-thirds full. For a layered effect, pour it over the back of a spoon so it settles gently.
Slowly pour the blue tea over the milk, again using the back of a spoon if you want distinct layers. The tea will naturally create an ombré effect as it mingles with the milk.
If using vanilla extract, stir it in gently. For a color-changing experience, squeeze a few drops of lemon or lime juice into the drink and watch it shift from blue to purple-pink. Serve immediately with a straw.
