Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan.
In a saucepan combine 250g margarine, 1/4 cup cocoa, water, eggs and 1/2 cup buttermilk in a saucepan. Cook, stirring occasionally, until it bubbles. Remove from heat and set aside.
In a large bowl, mix flour, 2 cups sugar and bicarb soda. Make a well in the centre and pour in chocolate mixture. Add vanilla and beat until smooth. Spread batter into prepared pan.
Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.
In a small bowl, mix peanut butter and oil. Spread over top of cake. In a saucepan, combine 1/4 cup cocoa, 125g margarine and 6 tablespoons buttermilk. Heat until boiling. Remove from heat and stir in vanilla. Place icing sugar in a large bowl. Beat in chocolate mixture and continue mixing until smooth. Spread over cake.