Preheat oven to 350 degrees. Rinse potatoes; then prick skins with a fork. Arrange on a baking pan and slide into oven. Bake for 45 minutes, or until potatoes are completely soft and inside is fork-tender. Remove from oven and let cool for 10 minutes or until cool enough to peel. Peel potatoes with a sharp knife and place into a mixing bowl. Mash the sweet potatoes along with the butter and pour in the syrup and a pinch of salt and pepper.