Preheat the oven to 350 degrees F (180 degrees C). Spray coating of culinary or lightly grease the bottom and sides of a mold (Mold to cake rolls) 15 x 10 x 1 inch (38 x 25 x 2.5 cm) with shortening. Combine all the ingredients of the bars with the exception of the raisins in a large bowl; beat in the electric mixer for 30 seconds at low speed, then during other 2 minutes at medium speed, scraping the bowl from time to time. Incorporate the raisins. Pour into the mold. Bake for 20 to 25 minutes or until the color slightly golden. Leave to cool completely on a grid for about 2 hours. Spread the icing on the bars. Sprinkle with walnuts. To get the bars, divide in 7 rows of 7 columns. Keep in the refrigerator.