Preheat oven to 400° F. Lightly grease a 9x13 pan.
Sift the sugar and flour together in a bowl. Melt the butter on low in a saucepan, and when melted add cocoa and water. Turn the heat to high and while stirring, heat until boiling. Pour the cocoa mix over the sugar and flour and mix well with a spoon.
Add the eggs, buttermilk, vanilla, baking soda, cinnamon, chile powder, and salt, and mix well with a spoon.
Pour batter into the pan, and bake for about 25-30 minutes, or until an inserted knife comes out clean.
Meanwhile, 5 minutes before cake is done, to make the frosting, bring to a boil the butter, cocoa, and milk in a saucepan on low heat. Remove from the heat, and stir in the sugar, vanilla, cinnamon, pecans, and salt. Beat well, and then spread over cake while both are still warm.
Notes
My family always made this cake in a 9x13 pan, but if you wish to bake it in an actual half-sheet pan, I'd reduce the baking time by 5 minutes.