Preheat oven to 250 degrees F (120 degrees C). Grease an 8x8 inch baking pan.
In a large bowl, cream together the butter, brown sugar, and eggs until light and fluffy. Stir in the unsweetened chocolate and vanilla until well blended. Mix in the flour, and then the walnuts. Spread batter evenly into the prepared pan.
Bake for 1 hour in the preheated oven, or until a toothpick inserted into the center comes out clean. Cool in the pan on a wire rack before cutting into bars.
