Add a tablespoon or two of the green sugar to a shallow dish. Brush the edges of a cocktail glasses with the corn or simple syrup and then roll it around in the sugar. Place in the refrigerator for a few minutes to set and chill the glass.
Add the fresh mint and milk to a cocktail shaker and muddle well with a muddler or the back of a wooden spoon. Pour in the whiskey, Irish cream and a couple of drops of green food coloring, if you want to color the cocktail. Fill with ice, shake well and strain into the prepared glasses.