Grease 2 x 18cm (7in) sandwich tins and preheat oven to Gas 5,190°C, fan 170°C.
Cream together the margarine and sugar until light and fluffy.
Add the beaten eggs, one at a time.
In a separate bowl, mix the drinking chocolate, cocoa and boiling water to a paste.
Beat the chocolate mixture into the margarine, sugar and eggs. Once it is completely combined, stir in the flour.
Split the mixture between the two sandwich tins and bake for 25-30 minutes.
To make butter cream filling
Cream together the margarine and icing sugar until smooth and light. In a separate small bowl blend the cocoa and milk to a smooth paste. Add to the creamed icing sugar and margarine.
Spread roughly a third of the butter cream on the top of one of the cakes. Carefully place the second cake on top and with the remaining butter cream ice the top and sides.
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