Lightly butter a 9x5x2 3/4 loaf pan. Line with heavy-duty foil;lightly grease foil with butter or margarine. In heavy, 3-quart saucepan, cook brown sugar and cream over low heat, stirring constantly with a wooden spoon, until sugar dissolves and mixture comes to boiling. Cook, stirring occasionally, until candy thermometer registers 234F, or until a little of the mixture in cold water forms a soft ball. Remove from heat, and add butter, but do not stir. Set aside, without stirring, to cool to lukewarm (110F). (Warning, dont let any of the sugar crystals on the side of the pan get into your mixture after it is cooked, or the texture will become grainy) Add vanilla. With wooden spoon, beat until thick and creamy. Stir in nuts. Quickly turn into prepared loaf pan; cool completely; refrigerate. Turn penuche out of pan in one piece. Remove foil. You can stop here if you wish and just cut into pieces. To make walnuts: Take 1 well-rounded tablespoon of penuche. Using a real walnut as model, shape penuche with fingers into a walnut. Form an oval 1 1/2 inches long and tapered at one end. Then, using a toothpick, make line and indentations, as on the walnut. Refrigerate each walnut after it is made. If penuche is too soft to handle, refrigerate 1 hour. Makes 24