Place prepared sweet potatoes in a medium saucepan and add enough water to cover them, plus an extra inch.
Bring the water to a boil and cook the cubed sweet potatoes for about 15 minutes, or until they are very tender.
Drain the sweet potatoes, and place them back in the pot.
Add mascarpone, salt, and 2 tablespoons of the prepared Rosemary Brown Butter.
Using an immersion blender, puree ingredients until everything is smooth and creamy, with no lumps remaining. (You can do this step in a food processor if you don’t have an immersion blender.)