Let 1 pint vanilla ice cream sit out at room temperature until starting to soften, 10 to 15 minutes (softened ice cream will blend with ease and require less milk to get things moving). Give the container a squeeze — it shouldn’t be rock hard and should give slightly when it’s ready. Meanwhile, chill the glasses.
If you have room in your freezer, stick 2 tall drinking or milkshake glasses in there, or else chill the glasses in the refrigerator. This will ensure the milkshake stays cold as long as possible.
Place 2/3 cup whole milk, 1/2 teaspoon peppermint extract, 8 drops liquid or 4 drops gel green food coloring , and the softened vanilla ice cream in a blender.
Starting on the lowest speed and working your way up to medium speed, blend until the milkshake is smooth and pourable, about 1 minute. Stop the blender and scrape down the sides or stir as needed to keep things moving.
Divide the milkshake between the glasses. Garnish with whipped cream, green sprinkles, and 2 maraschino cherries if desired. Serve immediately.
