Preheat the oven to 180 degrees C. Grease a 20x30cm baking dish.
In a large bowl, cream together the butter, copha and sugar until light and fluffy. Beat in the eggs one at a time, mixing well after each one. Combine the flour, baking powder, bicarb soda, nutmeg and salt; stir into the creamed mixture alternately with the buttermilk until just blended. Pour the batter into the prepared pan.
Bake for 40 minutes in the preheated oven until the cake springs back when pressed lightly in the centre. Cool completely before icing.