How to Make It
Place russet potatoes in a large bowl; cover with water to 1 inch above potatoes.
Place Yukon Gold potatoes in a large saucepan; cover with water to 2 inches above potatoes. Bring to a boil over high heat. When water reaches a simmer, drain russet potatoes, and add to pan with Yukon Golds. Bring to a boil; reduce heat, and simmer 12 minutes or until potatoes are very tender. Drain.
Using a potato ricer, rice potatoes over pan. Stir in butter until melted. Gently stir in 1 cup plus 2 tablespoons buttermilk, salt, and black pepper.
While potatoes simmer, bring chicken stock, sage leaves, and butternut squash to a simmer; cook 15 minutes. Reserve 2 tablespoons stock; drain squash. Discard sage. Place 2 tablespoons buttermilk, 2 tablespoons stock, and squash in a blender; blend until smooth. Add squash mixture to mashed potatoes; swirl with a spoon.