Heat oven to 350 degrees F. Spray 3 (9 by 2-inch) round cake pans with cooking spray, and line with waxed paper. Set aside. In a medium bowl, sift together flour and baking soda. Set aside.
In a medium bowl, combine sugar, vegetable oil, eggs, vinegar, and vanilla. With an electric mixer, beat until light and fluffy, about 2 minutes. Gradually add flour mixture, and mix on low speed just until the flour has been incorporated. Slowly add buttermilk. Add food coloring, and beat to combine.
Divide batter between pans; each pan will be about half full. Tap pans on counter to remove bubbles. Bake until a cake tester inserted into the middle comes out clean, about 30 minutes. Remove the pans to a rack to cool for 5 minutes. Invert pans onto wire racks sprayed with vegetable oil to cool cake completely, or quick-chill in the freezer for 10 to 15 minutes.
To assemble, place 1 layer, top-side down, on a cake stand. Using an offset spatula, spread with 1/4-inch of frosting. Repeat with remaining layers. To frost the top and sides of the cake, work from the center toward and over the edge, making sure to evenly coat. Store in an airtight container for up to 1 week.
Place cream cheese and margarine in a medium bowl. With a handheld electric mixer, beat until light and fluffy, about 2 minutes. Add sugar, pecans, and vanilla. Beat, on low speed to combine. If too soft, chill until slightly stiff, about 10 minutes, before using.
Yield: 6 cups
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