Add the Thai tea mix into a tea filter bag. Double-bag it as the tea leaves are quite fine and can easily leak into the liquid. If you don't have tea filter bags, prepare a fine sieve instead.
In a saucepan, add oat milk (or whole milk) and condensed milk. On medium-low heat, stir until the condensed milk is fully incorporated.
Add the teabag to the saucepan. Stir occasionally. Let it steep until steam begins to rise from the milk. Do not boil the milk.
Remove the saucepan from heat and set aside with the lid on for another 15 minutes.
Meanwhile, you can prepare the brown sugar pearls as per package instructions. (optional).
Once ready, use a spatula to gently press the teabag so the milk can absorb all the tea flavor. Remove the teabags from the milk and pour the milk tea through a fine sieve into a jug, cup, or small bowl that you want to pull tea with.
Pour the tea back and forth between the saucepan and the jug at least 5 times.
Add the whipping cream or evaporated milk (optional).