I think they mean brown malt. That's certainly what it sounds like. Note that the method - finishing kilning very quickly and at a very high temperature - cannot be the same as that practised in the 18th century as all the diastase was destroyed. Whereas in the 1700's brown malt was a base malt, often used by itself.
The form of the genuine porter malt-kiln differs from the common description for drying pale malt. The floor of the former is laid with tiles in the usual manner where such are used. The fire-chamber beneath is built with brick, within the square apartment, in the shape of an inverted pyramid, in the apex of which the furnace is placed. The furnace is arched with fireibrick, and extends 2.5 feet within the chamber, to disperse the heat equally to the floor above.
The malt to be prepared for porter-brewing is half made in the usual manner for drying pale malt. It is then divided into two or three parts, which are dried and finished on the kiln at such a high temperature as speedily turns it of a brown-colour, but without scorching or charring it; and converts it into porter malt.
It is first dried with coke in the usual manner. Birch-cuttings, or beech, when the former cannot be procured, are prepared to blow it, as it is termed, on the kiln, and give it the brown colour and that bitter principle which is so desirable to the taste in the consumption of porter.
When the malt is spread on the kiln-floor, the furnace is gradually charged with the wood-cuttings until a temperature upwards of 200° is obtained. It is carefully watched by the maltster, until it begins to burst by the escape of the air confined between the kernel and husk of the grain. It is now turned by the maltster and his assistants with shovel and broom, working it quickly, and sweeping each division, as it is proceeded with; and this process is repeated until it is judged sufficiently brown for its purpose.
By this incipient charring its germinating principle is destroyed, and it loses the capacity of yielding sugar, by mashing, in the proportion of twenty per cent to pale malt made from the same description of barley."
"Brewing and Distillation" by Thomas Thomson and William Stewart, 1849, pages 278-279.
Now all I need is an 18th-century account with a similar level of detail.