Preheat oven to 180 degrees C. Grease and flour a 23cm tin. Sift together the flour, cocoa and baking powder. Set aside.
In a large bowl, cream together the margarine and sugar until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Pour batter into prepared tin.
Bake in the preheated oven for 40 to 45 minutes, or until a skewer inserted into the centre of the cake comes out clean. Allow to cool.