Burnt cork is a homemade cosmetic created by burning cork with some accelerant, such as alcohol. The resultant ash is applied directly to the skin, or ground and mixed with some vehicle such as water. Because the pigment produced is carbon black, a known carcinogen, this is no longer considered a safe cosmetic product.
Burnt cork is probably best known in it's use to create blackface. This was demonstrated in the movie Bamboozled.