Grind the Thai tea mix into a very fine powder using a spice grinder or blender.
In a medium bowl, whisk together the flour, Thai tea powder, cornstarch, baking soda, baking powder, and salt.
In a large bowl, whisk together the partially melted butter, softened butter, brown sugar, and granulated sugar until smooth and glossy.
Add the egg, egg yolk, vanilla extract, and sweetened condensed milk. Mix until fully combined.
Fold the dry ingredients into the wet ingredients just until combined.
Fold in the chopped white chocolate and frozen mini marshmallows.
Cover the dough and chill for 1 hour.
Preheat the oven to 350°F. Line baking sheets with parchment paper.
Scoop large portions of dough using about 3 tablespoons each and place them 3 inches apart on the prepared baking sheets.
Bake for 11 to 12 minutes or until the edges are golden brown and the centers still look slightly soft.
Let the cookies cool on the baking sheet for 10 minutes before transferring to a cooling rack.