How do I make food look burnt?

> using a card with a hole in it

...or using your hand forming a hole with your fingers - as Photoshop burn tool icon suggests! 😉

Just FYI, "burn" refers to an old darkroom technique where you give added exposure to a part of the print, using a card with a hole in it. It is not something a chef would use to flambé a dish of coq au vin.

As Mylenium correctly indicates, this would be a very complicated and time-consuming task. You'll need to completely alter all the tonal relationships in the image, effectively constructing a new image.

Depends on the image, but there are no shortcuts for realism. This could be extremely complicated, especialyl since different types of food naturalyl show different scorching/ burning patterns and colors. If this is for a commercial project, you might have better luck creating it physically by charring some veggies with a blow torch in a fireproof studio environment... in any case, you mustn't mistake the Burn tool (or blending mode) for actual burning. This term is merely derived from the color changes actual burning/ melting of old chemical film would do to the photos on it. That's were the relation to actual fire ends.

Mylenium