Place potatoes, cauliflower florets, and smashed garlic in a large pot. Cover with cold water by 2 inches and add 1 tablespoon kosher salt. Bring to a boil over high heat, then reduce to a simmer. Cook until both vegetables are completely tender when pierced with a fork, about 15-18 minutes. The cauliflower should be soft enough to break apart easily.

Drain the vegetables in a colander and let them sit for 2-3 minutes to release excess steam. This step is crucial - too much moisture will make your mash watery instead of fluffy. Return the vegetables to the hot pot off the heat for 1 minute to evaporate any remaining moisture.

Add the butter and cream cheese to the warm vegetables. Mash with a potato masher or pass through a ricer for the smoothest texture. The residual heat will melt the butter and cream cheese, creating a rich base. Mash until mostly smooth with some small lumps remaining for texture.

Pour in the warm milk and fold gently with a rubber spatula until fully incorporated. The mixture should be creamy and light, not stiff. If needed, add more milk 1 tablespoon at a time to reach your desired consistency. Season with 1/2 teaspoon black pepper and additional salt to taste.

Transfer to a serving bowl, make a shallow well in the center, and add a small pat of butter if desired. Sprinkle with fresh chives and serve immediately while hot.