Brew a strong chai concentrate. Steep 2 chai tea bags in 1 cup of just-boiled water for 5–7 minutes. Squeeze bags gently and discard.
Chill in the fridge or freezer for a few minutes to cool.
Make the cherry purée. Blend pitted cherries with 1 tablespoon sweetener and the vanilla until smooth. If using frozen cherries, let them soften a bit first. Add a splash of water only if needed to blend.
Taste and adjust. If the cherries are very tart, add another 1/2–1 tablespoon of sweetener.
For extra brightness, mix in 1 teaspoon lemon juice.
Prepare the glass. Fill a tall glass halfway with ice. Spoon 2–3 tablespoons of cherry purée around the sides and bottom for a swirled look.
Add the chai. Pour 1/2 to 3/4 cup of cooled chai over the ice, depending on how strong you like it.
Top with milk. Add 3/4 to 1 cup of your chosen milk. Stir gently to create a marbled effect, or mix thoroughly for a uniform pinkish-tan color.
Finish. Taste and adjust sweetness.
Dust with a pinch of cinnamon or cardamom if you like. Serve immediately.