Grease and flour an aluminum tube pan. Heat the oven to 350.
In a large mixing bowl on medium speed, cream together:
1 cup softened butter
2-3/4 cups sugar
1 tsp. vanilla
Beat in 6 eggs - one at a time - beating one minute after each addition, scraping sides of bowl often.
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Measure out 3 cups of flour and add 1/2 tsp. baking soda to it.
Have 1 cup sour cream measured in a cup.
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To the creamed batter mixture, alternately add the flour and sour cream - starting with flour and ending with flour - mix on low speed, scraping the sides of the bowl often.
FOR THE CHOCOLATE:
In a small bowl, mix 6 Tblsp. cocoa powder with 2 tblsp. vegetable oil and mix until smooth. Add 1-1/2 cups of the cake batter to the chocolate and mix together thoroughly.
Put several Tblsp. of plain batter into the bottom of the tube pan, then add several Tblsp. of chocolate batter - keep alternating the plain & chocolate until it's all in the pan. Take a butter knife and GENTLY swirl the batter briefly, to make a chocolate swirl effect.
Bake the cake for 90 minutes (1-1/2 hr) @350. Test it with
a toothpick to make sure the center is completely done. Remove from the oven - cool for a while before removing from pan. Cool completely. Frost the cake or dust it with powdered sugar.
