In a saucepan, place potatoes in cold water to cover by 1-inch. Bring to a boil, then turn down to medium and cook until tender, about 10-15 minutes. Drain potatoes and place in a food processor; add 3/4 cup chicken broth to potatoes and puree.
In the saucepan, add butter and melt. Whisk in flour, rosemary, salt, pepper and cook for one minute whisking constantly. Add milk all at once. Cook and stir till slightly thickened and bubbly. Cook 1 minute more.
Stir in potatoes and remaining 3/4 cup broth. Cook until heated through, stirring.