Let the vanilla ice cream sit at room temperature for 3-4 minutes to soften slightly. The ice cream should be soft enough to scoop easily but not melted - this ensures a thick, creamy shake texture rather than a thin, icy one.
Add the softened vanilla ice cream to your blender first, followed by the cold whole milk. Starting with ice cream at the bottom helps the blender blades grip and blend more efficiently, preventing chunks of unblended ice cream.
Add the peppermint extract to the blender, being careful not to add too much as peppermint flavor intensifies quickly. Start with 1/4 teaspoon - you can always add more, but you can't take it away once it's too strong.
Add 6 drops of green food coloring to start, which will give you that signature pale green Shamrock Shake color. The shade should be a soft mint green, not dark or artificial looking - add more drops one at a time if needed.
Secure the blender lid tightly and blend on medium speed for 30-45 seconds until the mixture is completely smooth and creamy. You should hear the motor change to a steady whir when everything is fully incorporated, with no ice cream chunks remaining.
Stop the blender and check the consistency - it should coat the back of a spoon and slowly drip off, similar to a thick smoothie. If it's too thick, add 1-2 tablespoons more milk and blend for 10 more seconds.
Pour the shake into a tall glass, leaving about an inch of space at the top for whipped cream if desired. The shake should pour in a steady, thick stream - if it's too thin, you can pop it in the freezer for 5 minutes to thicken up.
Top with a generous swirl of whipped cream and place a maraschino cherry on top for the classic McDonald's presentation. Serve immediately with a wide straw while the shake is still cold and thick for the best flavor and texture experience.