In a canning jar (or any similar jar with a good seal), add the 6 oz. of water and instant coffee granules. If using a sweetener, add it as well.
Seal the jar securely and shake vigorously for several seconds to dissolve the coffee and to generate a thick foam.
Add ice cubes to fill about ¼-1/3 of the glass. Pour in the milk.
Give the jar one more quick shake before slowly pouring the coffee into the glass.
Notes
A mason or canning type jar is a perfect jar to use since the seals on the lid are quite good. I re-used a pasta jar which also happened to be a canning jar!
Depending on the size of your drinking glass, you may need to play with the proportions of ice, milk, and coffee.
Don't completely fill the glass with ice as it will make the glass too full and you won't get the desired 3-layer effect.
If using a sweetener, a simple syrup or liquid sweetener would work best. Otherwise, if using a dry sweetener, the finer the granules, the better.
Adjust the amount of coffee solution and milk and sweetener to your tastes.
If you don't drink dairy, try it with soy, nut, coconut, or other non-dairy milks!
For a slightly richer coffee, use evaporated milk instead of regular milk.
Top it with whipped cream or cocoa for an extra special touch!