Add cold brew coffee, vanilla ice cream, milk, and vanilla extract (if using) to a blender. Tip: For a thicker shake, freeze your glass for 10 minutes first.

Blend on high speed until smooth and creamy, about 30 seconds. Tip: Stop and scrape down the sides if needed.

Drizzle chocolate syrup inside serving glasses (if using), then pour the milkshake into glasses. Tip: Swirl the syrup for a marbled effect.

Serve immediately with a spoon and straw. Tip: Top with whipped cream or coffee beans for extra flair.