Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan. In a saucepan melt 1 cup of butter. Stir in 1 cup (250ml) water and 4 tablespoons cocoa powder. Remove from heat and set aside.
In a medium bowl, dissolve the bicarb soda in 1/2 cup of buttermilk. Stir in the eggs and vanilla. Mix in the cooled cocoa mixture.
In a large bowl sift together the flour, sugar, cinnamon and salt. Make a well in the centre and pour in the cocoa buttermilk mixture. Stir until blended. Pour batter into prepared pan. Bake in the preheated oven for 30 minutes or until a toothpick inserted into the centre of the cake comes out clean. Ice the cake with Buttermilk Pecan Icing while it's still warm.
For the icing: In a large saucepan melt 125g butter with 4 tablespoons cocoa. Stir in 1/3 cup buttermilk and heat until almost boiling. Stir in icing sugar, vanilla, salt and pecans. Remove from heat. Mix well and pour over warm cake.