Sift the sugar and flour together in a bowl.Melt the butter on low in a saucepan, and when melted add cocoa and water and heat until boiling. Pour cocoa mix over sugar and flour and mix well with a spoon.
Add eggs, buttermilk, vanilla, baking soda, cinnamon, chile powder, and salt, and mix well with a spoon.
Pour batter into greased 9×13 inch pan, and bake for about 30 minutes, checking it at 20 minutes.
Meanwhile, 5 minutes before cake is done, bring to a boil the butter, cocoa and milk. Remove from heat, and mix in sugar, vanilla, cinnamon, pecans, and salt. Beat well, and then spread over cake while both are still warm.
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.