Heat butter in a large heavy skillet over medium-high heat. Saute mushrooms until soft and lightly browned, about 2 minutes.
Stir in port wine, and let simmer for 2 minutes; remove from heat.
In a small bowl, mix together sour cream and cornstarch until smooth. Stir into mushroom mixture. Return to heat, and stir until thickened and smooth. Season to taste with salt and pepper.