In a small saucepan, melt butter over medium-low heat. Whisk in flour and let cook 1-2 minutes, stirring frequently.
Gradually stir in chicken broth, then milk, whisking constantly. Let cook until bubbly and thickened, whisking frequently. Season with salt and pepper.
Note: For cream of celery soup, use 1 teaspoon celery salt in place of salt.