Whats the best way to make a circular piece of ice for a glass of liquor?

In places like Japan, glasses of scotch (or other liquor) when served “on the rocks” will often have a big piece of ice shaped into a perfect circle. I assume this is made by shaping the circle from block ice. Is anyone aware of any other way to create this type of ice. It makes for a great glass of scotch? The ice does not melt like when served in cube form.