Preheat oven to 350 degrees F. Heat Dr. Pepper with margarine, but do not boil. Set aside. Sift together flour, sugar, cinnamon, salt, and cocoa. Beat together eggs, buttermilk, vanilla, and baking soda. Add hot Dr. Pepper mixture to dry ingredients. Stir in egg mixture and blend well. Pour into greased and floured 15x10-inch sheet cake pan. Bake for 25 minutes. For the Frosting: About 5 minutes before cake is done, heat Dr. Pepper, margarine, and cocoa. Stir in powdered sugar, pecans, and vanilla. Spread frosting on while cake is still hot.
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