Pour 1 cup of water into the bottom of your Instant Pot and place the trivet (rack) inside. Grease a 7-inch round cake pan or similar Instant Pot-safe baking vessel thoroughly with butter or cooking spray, then wrap the bottom and all four sides tightly with aluminum foil. This prevents water from seeping into the batter during cooking. Set the prepared pan aside while you make the batter.

In a large mixing bowl, combine the softened butter and sugar, then beat on medium speed for 2-3 minutes until the mixture is light and fluffy. This creaming process incorporates air into the batter, which helps the casserole rise and develop a tender crumb. Stop occasionally to scrape down the bowl.

Add the room temperature egg to the butter mixture and beat for about 1 minute until fully incorporated and the mixture looks smooth. Then add the sour cream and beat for another minute until well combined. I like to use sour cream at room temperature because it blends more smoothly into the butter mixture, creating a silkier batter base.

Add half of the cornbread muffin mix to the wet ingredients and beat on low speed until just combined, about 30 seconds. Pour in the milk and mix briefly, then add the remaining cornbread mix and beat on low speed until the ingredients just come together into a smooth batter. Avoid overmixing, as this can make the casserole dense. Stop as soon as you don't see any dry streaks of mix.

Using a spatula or wooden spoon, gently fold the whole kernel corn and creamed corn into the batter with just a few strokes—don't overmix. Pour the batter into the foil-wrapped baking pan and smooth the top. Tent a piece of aluminum foil loosely over the top of the pan (don't seal it tightly), then carefully lower the pan onto the trivet in the Instant Pot. I fold in the corn gently because the whole kernels are delicate and you want them to stay intact throughout cooking.

Close the Instant Pot lid and set the valve to sealing. Cook on high pressure for 50 minutes. When the timer beeps, carefully perform a quick release by moving the pressure valve to venting (use tongs or a wooden spoon if needed to avoid steam burns). Remove the lid by tilting it away from you, then carefully lift out the trivet and pan using kitchen mitts or tongs.

Remove the tented foil from the top of the pan and let the casserole rest in the pan for 15 minutes. This resting time allows the casserole to set and firm up slightly, making it easier to serve. You can serve it directly from the pan, or run a knife around the edges and invert it onto a serving plate if you prefer.