Melt butter in a stockpot over medium heat; add onions, bacon, and bay leaf. Cook and stir until onions are nearly transparent, 5 to 7 minutes.

Pour chicken broth into the pot; add potatoes, flour, marjoram, caraway seed, salt, and pepper. Bring to a boil, reduce heat to medium-low, and simmer until potatoes are tender, about 20 minutes.

Stir sour cream into soup; cook another 1 to 2 minutes.