McDonalds Shamrock Shake Rip-off

Reports say that the Shamrock Shake at McDonalds is a ripoff and that McDonalds copied the recipe from someone
Here is the recipe to make free Shamrock Shakes :
McDonald's Shamrock Shake Copycat Recipe 
(makes 1 giant shake, or 2 generously sized shakes) 

Ingredients: 
3 cups good quality vanilla ice cream 
1 3/4 cups 1% milk 
1/2 teaspoon peppermint extract 
Green food coloring, if desired 
Directions: 
Blend all ingredients in a blender until completely thick and smooth. Pour into glasses and serve!