Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
In a large bowl, cream together the butter, shortening, and sugar until light and fluffy. Beat in the eggs one at a time, mixing well after each one. Combine the flour, baking powder, baking 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 center. Cool completely before frosting.
