Put ¾ cup of the broth in a large pot. Cover and bring to a boil over high heat.
Add the corn, cover, decrease the heat to medium, and cook until the broth has evaporated, about 5 minutes.
To make the cilantro butter, put the cilantro, cashews, and remaining ¼ cup of broth in a small food processor or small blender and process for 1 minute.
Smear each ear of corn with the cilantro butter and sprinkle with cayenne if desired.