Preheat oven to 180 degrees C. Grease and flour two round 23cm cake tins. Sift together the flour, bicarb soda and salt. Set aside.
Combine cocoa and boiling water in small bowl to form a smooth paste. Cool slightly.
In a large bowl, cream together the margarine, sugar and vanilla until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated. Mix in the cocoa paste.
Divide batter into prepared cake tins. Bake in the preheated oven for 35 to 40 minutes or until a skewer inserted into the centre of the cake comes out clean. Cool 10 minutes in tins then turn out onto a wire rack and cool completely.