Preheat oven to 350 degrees F (175 degrees C, gas mark 4). Grease and flour pan. Mix shortening and sugar in large bowl until thoroughly blended. Add egg whites to shortening mixture, reserving yolks, and beat an additional two minutes or until light and fluffy. Beat in pudding mix, peppermint extract and sour cream (mixture may look curdled). Sift flour, salt and baking powder together. Slowly add about half of flour mixture to shortening mixture. Add ¾ cup milk, then last of flour mixture, scraping bowl after each addition. Increase speed and beat until thoroughly combined, 2-3 minutes. Remove two cups of batter, stir in white chocolate chips. Spoon white chocolate batter into prepare pan. Add reserved yolks, cocoa; fudge pudding mix and remaining ¼ cup milk to rest of batter in bowl. Beat until completely combined. Stir in chocolate chips. Layer chocolate batter over top of white batter. Gently tap the filled pan on the counter a few times to remove air bubbles and pockets. Place pan on cookie sheet in the oven. Bake 50-60 minutes or until tester inserted in center of cake comes out clean. Cool in pan ten minutes and invert onto cooling rack. Dust with powdered sugar, if desired, and serve.