Step 1Heat the oven to 400 degrees. Use 1 tablespoon melted butter to grease the sides and bottom of a 4-cup souffle dish.
Step 2Combine the remaining warm melted butter with the yams in a mixing bowl and mash until smooth. Add the thyme and salt and pepper to taste and mix well.
Step 3Beat the egg whites in a separate bowl until stiff peaks form. Stir one-fourth of the beaten egg whites into the sweet potato mixture to lighten it, then gently but thoroughly fold in the remaining egg whites. Scrape the mixture into the prepared dish and sprinkle the cheese evenly over the top.
Step 4Bake 30 minutes, until puffed and browned. Serve immediately or soon after (the souffle will be denser and deflated after it sits).