Remove the outer leaves and core from the head of cauliflower and discard. Chop the cauliflower coarsely and put it in a medium saucepan. Cover with water 1 teaspoon of salt; bring to a boil over high heat. Reduce the heat to medium-low, cover the pan, and cook for about 10 to 12 minutes, or until it is very tender. Drain in a mesh sieve and set aside.
Meanwhile, clean the leeks. Cut the root end and dark green tops off the leeks and discard. Cut the leeks in half lengthwise and rinse under cold running water to loosen and remove any bits of sand within the leaves. Slice the rinsed leeks thinly.
Place the butter in a large skillet and place it over medium heat. When the butter stops foaming, add the leeks. Reduce the heat to medium-low and cook the leeks for about 7 to 10 minutes, or until the are very soft but not browned. Stir them frequently. You might have to lower the heat more, depending on your burner.
Add the minced garlic or garlic powder to the leeks and continue cooking for 2 minutes longer.
Put the drained cauliflower and leek mixture in a food processor and add the heavy cream. Process until smooth. Taste and add salt and freshly ground black pepper. Alternatively, process the mixture using an immersion blender or an electric mixer.
Spoon the mashed cauliflower into a serving bowl and enjoy.