Place two tall glasses in the freezer to chill while you prepare the shake.
Add the vanilla ice cream to a blender.
Pour in the cold whole milk and add the chocolate syrup.
Blend starting on low, then increase to high until smooth, thick, and creamy, 20–30 seconds; add 1–2 tablespoons more milk if needed to reach a pourable thickness.
Pour the milkshake into the chilled glasses.
Top each with whipped cream and a maraschino cherry. Serve immediately with straws.