Directions

1. Before you begin, bring all of the cold ingredients to room temperature: set out the sour cream, butter, and the eggs (separate the eggs while they are still cold) for about half an hour before making the cake batter.
2. Preheat an oven to 325 degrees F (165 degrees C). Grease and lightly flour a 10-inch tube pan. Sift together the flour, baking soda, salt, and cocoa powder. Combine the sour cream and vanilla extract in a separate bowl.
3. Beat the butter with an electric mixer in a large bowl until creamy, about 30 seconds. Gradually add 2 cups of sugar and beat on high speed until light and fluffy. The mixture should be noticeably paler in color. Add the room-temperature egg yolks one at a time, allowing each yolk to blend into the butter mixture before adding the next.
4. Pour in one third of the flour mixture and mix on low speed until combined. Blend in half of the sour cream mixture. Continue adding the flour mixture alternately with the sour cream, mixing until just incorporated. Fold in the chocolate chips.
5. Beat the egg whites and cream of tartar in a clean mixing bowl with clean beaters until soft peaks form. Gradually add the remaining 1/2 cup sugar and beat until medium-stiff peaks are formed (lift your beater or whisk straight up: the tip of the peak formed by the egg whites should curl over slightly.) Use a whisk or rubber spatula to fold 1/3 of the meringue mixture into the cake batter to lighten it.
6. Fold the remaining egg white mixture into the batter until combined. Pour the batter into the prepared pan and spread it evenly. Bake the cake for about 1 hour and 15 minutes, or until the cake springs back when you touch it lightly and a toothpick inserted in the cake comes out clean.
7. Let the cake cool in the pan for ten minutes before inverting the cake onto a wire rack to cool completely.