Melted brown sugar and sour cream gives this fudge its unusually good flavor. Melt butter in a heavy saucepan. Add brown sugar and heat to boiling. Add sugar and sour cream. Cook over medium heat until sugar dissolves, then slightly higher heat to 236 degrees F. Without stirring cool at room temperature to lukewarm. Beat until mixture holds its shape and loses its gloss. Quickly add vanilla and nuts. Spread immediately in a buttered 8-inch square pan. Cool and cut into squares. Garnish with walnut halves. Makes about 49 pieces.