Soak the corn cobs in water, so the husks don't burn when you grill them. Minimum 1 hour but overnight is fine.
Preheat the oven to 120°C.
To make the butter, put the pasilla chilli peppers on a baking sheet and bake them until very brittle, about 15 minutes. When they are done, remove and cool them. Remove the seeds and crumble the chillies into a food processor. Add the butter and process until combined.
Lay out a sheet of plastic wrap on top of a sheet of foil. Turn the butter out onto the wrap and roll it into a cylinder. First wrap the plastic wrap around the cylinder and then secure the foil around it. Twist the ends, gradually applying more pressure as you twist, in order to make the cylinder taught. Alternatively, pile the butter into ramekins and cover with plastic wrap. Refrigerate it until firm.
Preheat a grill or grill pan to medium-high heat. Remove the corn from the water and shake or pat them dry. Grill the corn on the cob for 10 minutes, turning it every couple of minutes. Remove the corn cobs from the grill and pull the husks back on each one. Lay slices of the chilled pasilla butter on and let it melt. The corn will have beautiful flecks of chilli. Or serve the ramekins on the dinner table for everyone to help themselves.