Butter a 8x8x2 inch baking pan; set aside. Butter the sides of a heavy 2-quart saucepan. In saucepan, combine the sugar,sour cream and butter. Cook on medium-high to boiling, stirring constantly with a wooden spoon. Clip candy thermometer to side of pan. Reduce heat to medium. Continue approx. 7 minutes, cooking to 238 degrees(soft ball stage), stirring occasionally. Mixture should boil at moderate, steady rate over entire surface.Remove from heat. Stir in chopped almond bark and vanilla until melted; add toasted walnuts.(Butter may seperate from cooked mixture, but beating reblends). Stir until smooth and creamy and mixture starts to thicken, about 3 minutes. Pour into prepared pan. Score candy while warm into 1" squares. Cool until firm. Cut into squares. Store in tightly covered container. Makes 2 pounds (64 pieces).