Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 15 minutes. Drain and transfer potatoes to a large bowl.
Melt ¼ cup butter in a skillet over medium heat; cook and stir green onions, jalapeno pepper, and garlic until soft and fragrant, 3 to 4 minutes. Mix green onion mixture and cream cheese together in a bowl; add remaining ¼ cup butter and stir until butter melts. Add sour cream and mix well.
Mix cream cheese mixture with the potatoes using a potato masher until most of the lumps are gone; season with salt.
