Preheat oven to 375 degrees. Grease bottom only of an 11x7 inch pan with butter.
In a mixing bowl, mix the flour, granulated sugar, salt and cinnamon.
Melt butter in a 3 ½ quart saucepan. Stir in the brown sugar, cocoa powder and water and bring to a boil. Pour the boiling cocoa mixture into the bowl with the flour mixture and stir until blended.
In the saucepan you used for the butter/cocoa mixture, stir together buttermilk, egg and vanilla. Add the egg mixture to the chocolate mixture and stir by hand until blended. Lastly, stir in the baking soda.
Pour batter into the pan and bake for 12 to15 minutes. Cake should spring back when touched.
While cake bakes, weigh out your confectioners’ sugar and sift it to remove any lumps. Set it aside.
Rinse the saucepan and melt the 4 tablespoons of butter. Add the salt and cocoa powder and stir over medium heat until smooth. Remove from heat and stir in the sour cream. Add the confectioners’ sugar about ½ cup at a time, stirring until smooth. If icing seems to be thickening you can put the pan over a low heat, but the residual heat from when you boiled the cocoa/butter mixture should be enough. Stir in the vanilla, then spoon over cake and spread quickly before icing starts to set.