Set a large pot over medium heat and add the butter. When the butter is melted, add the onion and cook for 3 minutes, stirring frequently, until softened.
Whisk in the flour, marjoram, caraway seeds, and black pepper.
Gradually add the vegan chicken broth, whisking continuously until smooth.
Add the cubed potatoes. Bring just to a boil then reduce to low heat. Simmer for 10 minutes, or until the potatoes are fork-tender. Remove from heat.
Whisk in the sour cream until fully incorporated. Adjust seasoning to taste, sprinkle with parsley, and serve hot.