In a large pan or dutch oven, place in the frozen sweet potatoes with 1/3 cup of water. Cover and place over medium heat until the sweet potatoes are soft and tender, stirring every few minutes.
Brown the butter by melting in a saucepan over medium heat until it starts to foam, and the milk solids start to brown.
To the sweet potatoes, add in the browned butter, cream, parmesan cheese, brown sugar, salt, and nutmeg. Use an immersion blender to cream the ingredients together to make a smooth mash.