Bring potatoes to a boil in a large covered pot of salted water. Reduce heat and simmer until tender when pierced, 15 to 20 minutes. Drain, return to pot, and stir to coarsely break up.
Meanwhile, in a medium saucepan over medium heat, cook butter and sage together, stirring often, until butter turns golden brown and sage is crisp but not dark, 10 to 12 minutes.
Pour three-quarters of sage mixture into potatoes and stir enough to loosely blend in butter. Transfer to a shallow bowl and drizzle with remaining sage mixture. Serve immediately.