Preheat the oven to 350 degrees Fahrenheit.

Cut the cauliflower away from the core and discard the core. Roughly chop the cauliflower florets into bite size chunks.

If using an Instant Pot: Add 1 cup of water to the Instant Pot along with the chopped cauliflower. Close the lid and set the valve to sealing. Set to pressure cook for 3 minutes. When the 3-minute cook time is finished, turn the valve to quick release. Once all of the pressure is released, open the Instant Pot and pour the cauliflower into a colander to drain off any excess water.

If using the stove top to cook the cauliflower: Place the cauliflower in a large pot and cover with water. Cover the pot with a lid and bring to a boil over medium-high heat. Reduce the heat to medium and simmer covered for 15 minutes, or until the cauliflower is very tender. Pour the cauliflower into a colander to drain.

Pour the hot cauliflower into a large bowl and add the butter, heavy cream, garlic powder, salt, and pepper to taste. Use an immersion blender to puree the cauliflower until smooth, or transfer to a food processor to puree.

Pour half of the cauliflower into a 9x12 glass casserole dish. Sprinkle with half of the shredded cheese. Top with the remaining cauliflower, then sprinkle with the rest of the cheese.

Bake the mashed cauliflower for 25-30 minutes at 350 degrees until the cheese is melted and the cauliflower is bubbling around the edges.

Serve hot with fresh chives if desired.