Brew the cold brew concentrate. Combine 1 cup coarsely ground coffee with 4 cups cold, filtered water in a jar or pitcher. Stir, cover, and steep in the fridge for 12–18 hours. The longer it steeps, the stronger it gets.
Strain the concentrate. Pour through a fine-mesh sieve lined with a coffee filter or cheesecloth.
Store the concentrate in a sealed container in the fridge.
Make the vanilla sweet cream. In a small jar, mix 1/2 cup heavy cream, 1/2 cup milk, 1–2 tablespoons simple syrup, and 1 teaspoon vanilla extract. Add a tiny pinch of salt if you like. Stir or shake until combined.
It should be pourable, not whipped.
Adjust to taste. For sweeter cream, add another teaspoon of syrup. For thicker texture, use more cream than milk.
Assemble the drink. Fill a glass with ice. Add 1/2 cup cold brew concentrate and 1/2 cup cold water (or adjust for preferred strength).
Pour 2–4 tablespoons of vanilla sweet cream over the top. Stir gently, or let it cascade for a marbled look.
Finish and serve. Taste and tweak. Add more cream for richness, more water for smoothness, or a splash of syrup if you want it sweeter.