Put the cubed sweet and russet potatoes together in a medium-to-large pot, and cover with cold water.
Bring the pot with potatoes and water to a boil on the stove. Reduce to a simmer and cook until the potatoes are tender and falling apart, 20-30 minutes. Be sure to test both russet and sweet potato cubes for done-ness.
Drain the water and return the potatoes to the pot. Using a potato masher, begin to mash the potatoes. Add the butter, salt, pepper, and apple cider vinegar.
When potatoes are close to the texture you want, add the sour cream and rosemary. Mix well to blend all ingredients evenly. Serve warm.
Note: these mashed potatoes are meant to be a bit on the chunky side. If you would like thinner mashed potatoes, you can add more sour cream, or a bit of chicken stock to obtain a smoother texture.