Place the chicken breast in a pot, cover with water, bring to a boil, then reduce heat and simmer for 15 to 18 minutes until cooked to 165°F / 74°C.
Remove the chicken, let it cool for 5 minutes, then shred or dice into bite-size pieces.
In a large bowl, whisk together mayonnaise, sour cream, lemon juice, garlic powder, black pepper, and salt until smooth and creamy.
Add the shredded chicken and diced celery to the bowl and toss until evenly coated.
Serve immediately on bread, croissants, or lettuce cups, or refrigerate for up to 3 days.