Melt 2 tablespoons butter in a large saucepan over medium-high heat; add mushrooms and shallots, and saute 3 to 5 minutes or until tender. Remove from pan.
Melt remaining butter in saucepan over medium-high heat; whisk in flour. Cook, whisking constantly, 1 minute. Whisk in chicken broth until smooth, and cook, whisking constantly, until slightly thickened. Stir in whipping cream and sour cream. Add mushrooms, shallots, salt, and pepper. Cook, stirring constantly, until thoroughly heated. Garnish, if desired.