Preheat oven to 180 C / Gas 4. Grease and flour a 23x33cm baking tin. In a saucepan, melt 225g of butter. Stir in 250ml water and 4 tablespoons cocoa powder. Remove from heat and set aside.
In a medium bowl, dissolve the bicarbonate of soda in 110ml 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 cake mixture into prepared tin. Bake in the preheated oven for 30 minutes, or until a skewer 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 remaining butter and cocoa. Stir in 75ml buttermilk, and heat until almost boiling. Stir in icing sugar, vanilla, salt and pecans. Remove from heat. Mix well and pour over warm cake.
How to ice a cake video
Watch our How to ice a cake video and see how to ice cakes with ease. You'll be icing perfect birthday and special occasion cakes in no time!