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Add the eggs to the creamed butter mixture one at a time, beating well in between additions. Beat in the dry ingredients and sour cream alternately, beginning and ending in flour. Add the vanilla.
In a separate bowl, combine the chocolate, walnuts, the remaining 1/4 cup sugar and cinnamon.
Sprinkle a third of the chocolate and nut mixture over the greased pan. Spoon in half of the cake batter. Sprinkle more of the chocolate and nut mixture over the batter, reserving some for the top. Pour the remaining batter into the pan. Smooth it out evenly and top with the remainder of the chocolate and nut mixture.
Bake for 45 minutes. Cool. Remove from the pan to a rack.