Preheat oven to 350 degrees. Brush an 11 x 7-inch pan with melted butter, or spray with non-stick spray. Line the pan with parchment paper, this will enable you to simply lift the bar cookies out of the pan.
Melt chocolate by either placing in a microwave and heating for 10-second intervals, stirring chocolate between 10-second intervals, or by placing chocolate into a glass bowl that has been placed into a pot of simmering water. Stir chocolate until it has melted. Place all ingredients into a food processor. Process until the mixture is smooth.
Spread batter into pan. Bake for 25 to 30 minutes or until a toothpick inserted center comes out clean. Cook 10 minutes before removing from pan. Heat chocolate for icing and sour cream in a small pan over low heat until the chocolate melts. Spread completely melted chocolate and sour cream over the cookies. Sprinkle with nuts if desired. After chocolate sets up, it takes about 40 minutes cut into triangles.