 The baked custard filling may vary from a light and silky to dense, depending on the recipe's ratio of mashed potato, milk and eggs.Though creamy vegetable pie recipes date back to Medieval Europe, sweet potato pie appears in the southern United States from the early colonial days.Like many sweet potato recipes, sweet potato pie was likely developed by African-American slaves from traditional African culinary influences as an alternative to the African yam, making it a staple of soul food today.