Most decriptions of how charcoal is made suggest that the most efficient way to turn wood (or other biomass) into charcoal is something called the "indirect method," where wood is put into an airtight but vented container, and heated; after a period of initial heating, burnable gasses start coming out
of the container's vents and can be used to finish the process without needing any additional fuel.
This use of gasses reduces the fuel requirements of turning wood into charcoal, but... in many descriptions, the fuel used to get the process going is wood, in an open fire underneath the container in which other wood is being turned into charcoal.
Burning wood in this manner is far from the most efficient way of getting heat from it: the best way to get heat from wood (that I know of) is to use an inverted downdraft gasifier... which also happens to produce charcoal as a byproduct of it's operation.
If one were to combine both technologies, charcoal making using the indirect method, and burning wood in an inverted downdraft gasifier, it should require less initial wood to produce a given amount of charcoal.