Preheat oven to 180 degrees C. Grease a Swiss roll tin.
In a large bowl, stir together the flour, sugar and bicarb soda. In a small saucepan, bring 375g butter, 4 1/2 tablespoons cocoa and 1 1/2 cups (375ml) water to a rapid boil. Pour over the flour mixture and stir until combined. Add the eggs, buttermilk and vanilla essence; mix well.
Spread batter evenly into the prepared baking pan. Bake in the preheated oven for 20 minutes. Cool before icing.
Icing: In a small saucepan melt 1/2 cup of butter. Stir in the icing sugar, 3 tablespoons cocoa powder, milk and 1 teaspoon vanilla, in that order. Beat by hand until creamy then spread onto cooled sheet of brownies. Cut into squares and enjoy.