In his classic book Kitchen Confidential, Anthony Bourdainsays this about demi-glace: “Freeze this stuff in an ice-cube tray, pop out a cube or two as needed, and your are in business – you can rule the world.” With praise like that from culinary institute graduate and resident bad boy of the culinary world, demi-glace must be some powerful stuff. Take our word for it; it is. But what is it?