Prep: Preheat the oven to 325 degrees F. Grease and flour your loaf pan(s) and set aside.
Cream butter and sugar: In a large bowl or the bowl of a stand mixer, cream together butter and both sugars until light and fluffy (about 5-7 minutes).
Add in eggs, and beat until just incorporated. Add in vanilla and beat once more. Do not overmix.
In a separate bowl, sift together flour, baking soda, and salt.
To the butter and sugar mixture, add a third of the sour cream and mix until just combined, and then add a third of the dry ingredients and mix until just combined. Repeat this process until all the sour cream and dry ingredients are combined into the batter.
Use a clean bowl, and divide the thick pound cake batter in half.
Add cocoa powder to one bowl of batter and mix until just combined.
In your prepared loaf pan, add dollops of vanilla batter and chocolate batter, alternating until the pan is 3/4 of the way full. Tap the loaf pan on the counter to release air bubbles. Use a skewer to create a swirled pattern all the way through the batter.
Bake in your preheated oven for 1 hour and 40 minutes or until a skewer comes out clean.
Video
Notes
Use real butter, not margarine, in this recipe for the best results.
If you do not have a large loaf pan, you can bake this pound cake in a bundt pan (same bake time), or two smaller loaf pans (reduce bake time by 20 minutes).
Nutritional information is approximate and was calculated using a recipe nutrition label generator and will vary based on cooking methods and brands of ingredients used.