Set the oven rack in the middle position. Preheat the oven to 350°F. Coat a 10-inch tube pan with vegetable oil spray or butter. Dust the pan with flour and tap the pan to remove the excess flour. 

Add the flour, salt, and baking powder to a  bowl and whisk to combine. Set aside.

Add the egg whites to the bowl of a stand mixer fitted with the whisk attachment and whip until they form soft peaks. Set aside. If you don’t have a second mixer bowl, transfer the egg whites to another bowl and set aside. The cake comes together quickly, so the beaten egg whites will not have time to deflate.

Add the butter and sugar to the bowl of the  stand mixer fitted with the paddle attachment and cream until combined. Add the egg yolks one at a time, and combine. Add the vanilla and beat in.

Add the dry ingredients alternately with the  milk to the creamed mixture and combine. Fold in the egg whites, and spoon half of the batter into the prepared pan. Gently shake the half-filled pan to evenly distribute the batter. Add the chocolate syrup to the batter remaining in the mixer bowl and fold in to combine.

Spoon the chocolate batter over the vanilla  batter in the pan and smooth the batter with a spatula. Do not mix the batters. Sprinkle the chopped chocolate and chopped walnuts over the top of the batter. Place the pan in the oven and bake for 60 to  65 minutes, or until a metal tester inserted into the cake comes out clean.

Transfer the cake to a wire rack and cool for  25 minutes. Run a butter knife gently around the edges and tube of the pan and invert the cake onto a second rack. Turn the cake right side up onto the first rack. Allow the cake to cool completely before slicing. Any leftover cake can be stored, covered  with wax paper, at room temperature, or stored in a cake keeper.

Recipe posted with permission from Baking with the Brass Sisters: Over 125 Recipes for Classic Cakes, Pies, Cookies, Breads, Desserts, and Savories from America's Favorite Home Bakers