Step 1: Build a charcoal fire and rake out the embers. Lay the corn in the husk directly on the embers. Char on all sides, until the husk burns through and you see golden brown kernels. This will take 3 to 4 minutes per side.
Step 2: Meanwhile, melt the butter.
Step 3: Transfer the corn to a heatproof serving platter. Using a stiff brush or knife, scrape off the burned husk and silk. Brush the corn with the butter. Sprinkle with barbecue rub and serve at once.