In a small Dutch oven, bring potatoes, broth, cream, salt, thyme, and pepper to a boil over medium-high heat. Cook, stirring occasionally, until potatoes are very tender and liquid has reduced, 15 to 20 minutes. Drain potatoes, reserving ½ cup broth. Add cream cheese and reserved broth; mash until liquid and cream cheese are absorbed into potatoes. Garnish with melted butter, if desired.