Combine sweet potatoes and 1/4 cup evaporated milk in food processor or blender; process until smooth. Add remaining milk, brown sugar, eggs, cinnamon, ginger and salt; process until well blended. Pour into 6 inch to 7 inch ( 1 1/2 quart ) soufflé dish or round baking dish that fits inside Instant Pot®. Cover dish tightly with foil.
Pour 1 1/4 cups of water into pot. Place soufflé dish on rack; lower rack into pot.
Secure lid and move pressure release valve to Sealing position. Press Manual and Pressure Cook; cook at high pressure 40 minutes.
When cooking is complete, use natural release for 10 minutes, then release remaining pressure. Uncover; let stand 30 minutes.
Remove soufflé dish from pot. Remove foil; cool 30 minutes. Top with whipped cream and nutmeg.