Place cubed potatoes in a large pot of salted water and boil for 10–15 minutes until fork-tender.
Drain and let potatoes cool slightly.
In a large bowl, mix sour cream, mayonnaise, garlic powder, salt, and pepper until smooth.
Add cooled potatoes and gently mix.
Stir in cooked chicken, shredded cheese, and green onions.
Mix until evenly combined, being careful not to mash the potatoes.
Cover and refrigerate for 1–2 hours to allow flavors to develop.
Garnish with extra cheese or green onions before serving.