Sweet Potato Purée with Streusel Topping

•Most American cooks use the terms "sweet potato" and "yam" interchangeably, but there are actually three different types of vegetable referred to by these terms: The true sweet potato (also known as a boniato), available at Asian and Latino markets, has pale yellow flesh, skin with a purplish cast, and a chestnutlike flavor that isn't sweet at all. The true yam, also called a ñame (NYAH-meh), is a large tuber with scaly brown skin that's a staple of Caribbean, African, and Hispanic cuisines. The familiar, bright-orange-fleshed tubers called for in this recipe are a third type, a variety of the sweet potato that are often called "yams" to distinguish them from yellow-fleshed "true" sweet potatoes. The Louisiana, jewel, or garnet varieties will all work well.
•While some recipes for sweet potato purée call for boiling the potatoes, roasting gives a much sweeter, more concentrated flavor and drier texture.