Adjust rack one-third up from bottom of oven; preheat oven to 325 degrees F. Grease a 10 by 4 inch tube pan. Line the bottom with a circle of wax paper cut to fit; grease the paper, then lightly flour the entire inside of the pan (including the tube), knocking out any excess. Set aside. Sift together cake flour, baking powder, and salt; set aside.
Melt the chopped chocolate in microwave or stove; set aside.
Fit a stand mixer with paddle beater, if available. In large mixer bowl, beat softened butter at medium speed until smooth. Add sugar and vanilla. Beat at a low speed until incorporated, then scrape beater(s) and bowl with rubber spatula (repeat this scraping often throughout the mixing process to ensure complete blending of ingredients). Increase speed to medium; beat for 3 full minutes. At a low speed, add eggs, two at a time. When all eggs have been added, increase speed to medium once more; beat for 1 minute (mixture may look curdled now — OK).
At lowest speed, add about half of sifted dry ingredients, mixing only until blended. Add orange liqueur. Add remaining dry ingredients and mix in, then remove bowl from mixer and fold in lemon zest with large spatula. Note: This is a large amount of thick, heavy batter. It may be necessary to increase mixer speed slightly to get dry ingredients to incorporate. I always end up removing the bowl from the mixer before the last of the dry ingredients are blended in completely, adding the lemon zest, then folding everything together with a large, sturdy rubber spatula. Batter may still look slightly curdled at this point — OK.
Measure out a generous 4 cups of the batter and set aside. To remaining batter in mixer bowl, add the melted chocolate (which may still be slightly warm) all at once; whisk in with large, sturdy whisk.
Place alternating spoonfuls of dark and light batters into prepared pan. To level batter in pan, grasp pan on opposite sides with both hands; twist briskly in short, back-and-forth motions. To marble, run a knife in a circular motion all around the batter in several concentric circles, going almost down to the bottom of the pan (don't overdo the marbling). With back of a large spoon, push batter slightly higher onto pan edges and tube, leaving a "trench" in the middle.
Bake in preheated oven for 1 hour and 15 minutes to 1 hour and 25 minutes. Turn pan back-to-front once about halfway through baking time. If top of cake begins to become too brown, cover loosely with aluminum foil. Cake is done when a toothpick inserted into the crack that will form all around the top emerges with a few moist crumbs still clinging to it. Do not overbake! My experience with this cake is that it will pull away from the sides of the pan only after it is removed from the oven.
Remove to cooling rack; allow to stand 15 to 20 minutes. Carefully loosen cake from pan edges and tube; invert onto another cooling rack (be careful! This is a large, tall cake). Re-invert to cool completely right side up. When completely cool, store airtight. Allow to stand at least overnight before serving. To cut, use a large, sharp, serrated knife.