Fill a medium sauce pan about halfway full with oil. Heat over medium high heat until hot. (I never measure with a thermometer. Instead, I test the oil with the handle of a wooden spoon. If the oil bubbles around the handle, I know it’s hot enough.)
Add a handful of sweet potato fries and cook until slightly browned and crisp, about 5-6 minutes/batch. Drain on paper towel or cheesecloth lined plate and sprinkle with salt immediately. Keep warm in an oven set at its lowest setting. Continue in batches until done.
For the sauce: Combine sour cream, brown sugar and pineapple in a small bowl. Mix well. Refrigerate until serving. This tastes fine served right away or prepared up to 6 hours in advance.