In a large pot of boiling, salted water, cook the cauliflower until very soft, 12 to 15 minutes; drain.
Meanwhile, in a medium skillet, melt 3 1/2 tablespoons butter over medium heat. Add the leek and garlic and cook, stirring occasionally, until the leek is translucent and very soft, about 8 minutes.
Using a food processor and working in batches, puree the cauliflower and the leek mixture with the cream.
In a small skillet, melt the remaining 1/2 tablespoon butter over medium heat. Add the panko and toast, tossing, until golden. Top the puree with the toasted panko and the parsley.