Heat evaporated milk in a saucepan over very low heat. Stir in Thai tea mix. Heat just until you see steam rising from the surface; do not boil. Turn off heat and steep for 30 minutes.
Strain tea mixture into a large bowl, pressing as much liquid as possible from the pulp. Rinse the pot and pour the mixture back in.
In a separate bowl, gently whisk egg yolks and set aside.
Return the pot to very low heat. Stir in condensed milk and heavy cream. Heat just until steaming; do not boil. Turn off heat.
Temper the eggs by slowly ladling the hot mixture into the egg yolks while whisking constantly. Repeat 1-2 times, then pour egg mixture back into the pot.
Return to low heat, stirring constantly until mixture thickens and coats the back of a spoon, about 3-5 minutes.
Let the mixture cool completely before churning in your ice cream maker according to manufacturer's instructions.
Transfer to a freezer-safe container and freeze for at least 6 hours before serving.