Place 4 heavy pint glasses in the freezer to chill for 30 minutes.
Add the ice cream, milk, ice cubes, bourbon and 4 tbsp of the chocolate syrup to a powerful blender.
Blend until smooth.
(A good milkshake should be eaten with a spoon, so if the mixture seems too thin, add another .25 to .5 cup ice cream and blend again.).
Divide evenly among the prepared glasses and top each with .5 tbsp of the remaining chocolate syrup.