The combination of butternut squash and starchy sweet potatoes creates an awesome texture, but if you want to just use one or the other, that works too. Double the butternut squash and make the recipe as directed - the result may be a bit more of a watery texture. Or triple the amount of sweet potatoes - you'll have to increase the amount of cream, or substitute with more whole milk, since the result will be thicker, and I would use a potato masher instead of an electric mixer otherwise the overworked starch may result in a gluey texture.