When mashed potatoes are on the menu, go for high-starch potatoes, such as russet, Idaho, or Burbank. The low moisture content in these potatoes makes them fluffy and ready to soak up plenty of milk and butter. Yellow medium-starch potatoes, such as Yellow Finn or Yukon Gold, make wonderful mashed potatoes, too.
If you already have potatoes but have no idea what kind they are, try this test: Slice one of the potatoes with a knife. If the knife becomes coated with a milky white substance, it’s a high-starch potato. If not, it’s a waxy—or low-starch—potato.