Scrape corn kernels from the cobs; run the back of your knife along the cob to extract all the corn liquid. Put corn into a food processor and pulse a few times to break up the kernels. Add masa harina and salt and pulse to incorporate. Heat the oil in a nonstick skillet over medium low heat. Divide the mixture into 4 and make 4 corn cakes. Cover and let the cakes cook for 5 minutes. Uncover the pan and carefully flip the cachapas over. Press on them with a spatula to flatten them a bit. Cook, covered, for another 5 minutes. Uncover and cook for about 5 more minutes; they should be a dark golden brown. Top with the cheese and cover until the cheese has melted. Serve immediately.