Combine 6 cups (1.4 L) water, tea bags, cardamom, cinnamon, sugar, and vanilla in a stove-safe tea kettle or medium pot.
Place over medium-high heat and bring to a boil.
Stir in the evaporated milk and return to a boil, watching carefully to ensure the tea doesn’t boil over.
Once just boiling, reduce the heat to medium-low and simmer until thick, creamy, and caramel in color, about 25 minutes.
Taste for desired sweetness and add more sugar to your taste.
Strain the chai through a fine-mesh sieve into mugs; discard the spices and tea bags.
Serve with additional sugar on the side.
