Prepare the cauliflower by removing the green leaves and trimming the core thoroughly to prevent a grainy texture, then cut into uniform 1-inch florets.

Melt the butter in a large Dutch oven over medium-high heat. Add a portion of the cauliflower florets—enough to cover the bottom in a single layer—and cook until they are deeply browned and fragrant, about 10 to 12 minutes. Remove the browned florets with a slotted spoon and set aside for the garnish.

Stir the leek, onion, and salt into the remaining butter in the pot. Reduce the heat to medium and cook until the vegetables are softened and translucent, about 7 minutes, being careful not to let them brown.

Add a portion of the remaining cauliflower and the water to the pot and bring to a simmer. Once simmering, stir in the rest of the cauliflower; cover and cook until the cauliflower is completely tender and breaks apart easily, about 15 to 20 minutes.

Process the soup in a blender until it is completely smooth and velvety. Work in batches to avoid overfilling the blender, then return the puree to the pot and adjust the consistency with a little extra water if it feels too thick.

Stir in the vinegar and season with additional salt and pepper to taste. Serve hot, topped with the reserved browned florets and chives.