Sift together flour, cocoa, baking powder and salt. Set aside.
Cream together butter, shortening and sugar in bowl until light and fluffy, using an electric mixer at medium speed. Add eggs, one at a time, beating well after each addition. Blend in vanilla (total beating time: 10 minutes).
Gradually add dry ingredients alternately with milk to creamed mixture, beating well after each addition, using low speed. Pour batter into greased 10” (25cm) tin.
Bake in 160 degrees C oven for 1 hour 20 minutes, or until a cake tester or toothpick inserted in centre comes out clean. Cool in tin on rack 10 minutes. Remove from tin. Cool on rack.
I use a chocolate ganache to ice this cake and between the layers.