Add all ingredients except for butter, cornstarch and milk into an Instant Pot.
Cook for 28 minutes at high pressure, then allow 10 minutes for natural release.
After cooking, shred chicken right in the pot. In a small saucepan, melt butter over medium heat. Add cornstarch and whisk for 30 seconds until smooth. Add in whole milk and whisk, stirring constantly until thickened. Add to soup and stir gently until all blended together and creamy. Serve.