For the Cake:
Preheat the oven to 350 degrees F.
Grease and flour 2 (9-inch) cake pans. In a medium bowl or on a piece of waxed paper, sift together flour, cocoa, baking soda, baking powder, and salt; set aside. In a large bowl, cream together sugar and butter. Beat in eggs, 1 at a time. Alternately add flour mixture and buttermilk. Beat in food coloring and vinegar, then add vanilla. Spread the batter evenly in the pans.
Bake for 20 to 30 minutes or until a wooden toothpick inserted into the center comes out clean. Turn out onto a rack to cool.
For the Frosting:
In a large bowl, combine the cream cheese and butter. Beat in confectioners' sugar until fluffy. Beat in vanilla. Stir in pecans. Use frosting to fill and ice cake.
Slice and serve on individual plates.