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 small bite-size pieces.
In a large bowl, whisk together mayonnaise, sour cream, sweet pickle relish, Dijon mustard, garlic powder, and black pepper until smooth.
Add the shredded chicken and diced celery to the bowl and toss until evenly coated.
Refrigerate for at least 30 minutes to let the flavors blend, then serve on bread, croissants, or lettuce cups.