So Moist Red Velvet Cake

Preheat an oven to 350 degrees F (175 degrees C). Grease and flour two 10 inch cake pans. Beat the shortening and white sugar with an electric mixer in a large bowl until light and fluffy. The mixture should be noticeably lighter in color. Room-temperature eggs should be added one at a time, allowing each egg to blend into the shortening mixture before adding the next. Mix the cocoa and red food coloring in a separate bowl and stir into the shortening mixture. Combine the flour and salt. Pour in the flour mixture alternately with the buttermilk, mixing until just incorporated. Stir in vanilla extract. In another separate bowl, mix the vinegar and baking soda. Fold in the vinegar mixture while fizzing; mixing just enough to evenly combine. Pour the batter into prepared pan. Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack before frosting. To make frosting: Cream together margarine and confectioner's sugar, then add vanilla extract. Beat until light and fluffy. Frost cooled cake and sprinkle with walnuts.