Preheat your oven to 400 degrees F. Line a baking sheet with parchment paper. Using a fork, prick the clean sweet potatoes in a few spots then place on the lined baking sheet. Bake until tender, about 45-60 minutes, depending on their size.
When the sweet potatoes are cool enough to handle, peel off the skin.
In a medium pan over medium-low heat, mash the peeled sweet potatoes with a potato masher. Add the coconut milk as you mash, starting with 1/4 of the can, and adding more as needed.
Season the mashed sweet potatoes with salt and pepper. Add a few pinches of cinnamon, if using.
Serve right away, or refrigerate until ready to serve. The mashed sweet potatoes can be reheated over medium-low heat. Add some of the extra coconut milk if needed.