In a large mixing bowl, add the following ingredients one at a time, beating well after each addition: eggs, caster sugar, 2 tablespoons butter, oil, 50g cocoa, buttermilk and vanilla.
Sift the flour, baking powder, bicarbonate of soda and salt; add to the sugar and egg mixture. Blend well and fold in the boiling water.
Place batter in two greased and floured 20cm round cake tins. Divide the chocolate chips in half and sprinkle them over each pan. Bake at 180 degrees C for 25 minutes. Cool.
To make the icing: Begin by beating together 125g butter, icing sugar and 50g cocoa; add 1 teaspoon vanilla, milk and coffee. Beat until very smooth. Add more icing sugar until you obtain the consistency you want. Spread on cooled cake.