To make the cake, sift together the sugar, flour, baking soda and cinnamon in a large bowl. In a saucepan, melt the margarine, then add the water and cocoa.
In a small bowl, beat the eggs with the buttermilk and vanilla. Mix everything together in the larger bowl and pour into a greased and floured 11-by-16-inch baking pan.
Bake at 400° for 25 minutes. Let cool for 20 minutes before icing.
To make the icing, bring the margarine, cocoa and milk to a boil in a pan. Remove from the heat and beat in the sugar and vanilla.
Pour half of the icing over half of the cake, add the pecans to the remaining icing and pour that over the other half of the cake.