Prick sweet potatoes with a fork in several places. Microwave on High until tender all the way to the center, 10 to 15 minutes. (Alternatively, place in a baking dish and bake at 425 degrees F until tender all the way to the center, about 1 hour.)
When cool enough to handle, peel off and discard skin. Transfer the sweet potatoes to a medium microwaveable bowl and mash thoroughly with a potato masher. Add coconut milk, ginger and salt; stir well. Reheat in the microwave for 1 to 2 minutes, or in the oven for 8 to 10 minutes. Serve warm.