Break chocolate into 1-inch pieces. Place in the bowl of a food processor, and pulse until chocolate is finely chopped.
Add nuts, sugars, and cinnamon. Pulse until nuts are finely chopped.
Preheat oven to 350°. Butter a 10-inch Bundt pan and dust with flour, shaking out the excess.
Stir together sour cream and baking soda in a small bowl. Set aside.
Whisk together flour, baking powder, and salt in a large bowl. Set aside.
Cut butter into 1-inch pieces. Mix on medium speed with an electric mixer for about 2 minutes. Add 1 cup of sugar, 1-2 tablespoons at a time, taking 4-6 minutes. Add egg yolks, two at a time, and beat for 30 seconds after each addition. Mix in vanilla.
Stir the milk into the sour cream mixture. Reduce mixer speed to low. Alternate adding the flour mixture (in 3 parts) and the sour cream mixture (in 2 parts), mixing just until blended after each addition.
In another bowl, beat egg whites on medium speed until frothy. Increase mixer speed to medium-high, and whip until soft peaks form. Add remaining 1/4 cup sugar, a tablespoon at a time, taking about 15 seconds. Beat for another 15 seconds.
Fold one-third of the egg whites into the batter, taking about 20 turns. Then, fold in remaining whites, taking about 40 more turns.
Spoon about one-third of batter into prepared pan. Sprinkle about one-third of the chocolate mixture evenly over the batter. Cover with half of remaining batter, and smooth evenly without disturbing the chocolate layer. Sprinkle with remaining chocolate mixture. Cover with remaining batter. Marbleize batter using a table knife cutting almost to the bottom of the pan.
Bake for 50-60 minutes, until top is golden brown and springy to the touch. A toothpick inserted in the center should come out clean. Cool on a wire rack for 20 minutes. Cover with a wire rack, invert the cake, and lift off the pan.
Store in an airtight container for 5 days. May be frozen.