In a medium deep skillet over medium heat, melt butter. Add shallots and sauté until soft, about 2 minutes. Add garlic and sauté for 1 minute. Add mushrooms, sprinkle a little salt and sauté until reduced by half their size. Add chicken broth, cognac and cream. Stir well, bring to a simmer and cook until sauce thickens, about 10 minutes.