Begin by prepping the cauliflower: peel off the outside leaves, then cut out the core carefully. Slice the whole head in half, then chop each half into 6 pieces. Place all pieces of cauliflower in a large glass bowl. Add the heavy cream, then cover the bowl with plastic wrap.
Microwave for 5 minutes. Carefully, remove from the microwave and peel back the plastic wrap (be careful! The steam is very hot). Stir the cauliflower, then add the salt and pepper. Cover again and microwave another 5 minutes.
When ready to serve, mash the cauliflower with a potato masher and the butter. For a very creamy puree, I use an immersion stick blender. A food processor is a great substitute, too.