2 lbs. sweet potatoes, peeled and chopped into 1-inch chunks
1 lb. Yukon gold potatoes, peeled and chopped into 1-inch chunks
10 tbsp. unsalted butter, divided
1-2½ tbsp. minced fresh sage, divided
1½ cups warm milk
Salt and pepper
1 cup fresh breadcrumbs
Combine the sweet potatoes and Yukon gold potatoes in a large stockpot. Cover with water, season with salt, and bring to a boil. Reduce the heat and continue to boil until the potatoes are tender, 10-14 minutes. Drain well and mash potatoes with a ricer or an electric mixer. In a saucepan, melt 8 tablespoons of the butter over medium heat, stirring occasionally, until golden brown, about 7 minutes. Remove from the heat and mix in 1-2 tablespoons of the sage. (I prefer to go a little light on the sage.) Mix the sage-butter mixture and the warm milk into the potatoes; mix well to blend until smooth. Transfer the mixture to a 2-quart casserole dish. At this point, the potato mixture can be covered and refrigerated for up to 2 days.
When ready to bake, preheat the oven to 375˚ F. In a small bowl, melt the remaining 2 tablespoons of butter. Mix in the remaining ½ tablespoon of sage and the bread crumbs; season with salt and pepper. Toss well to combine. Sprinkle the breadcrumb mixture over the sweet potato mixture. Bake uncovered until golden brown and bubbling, 30-40 minutes. (Cover loosely with foil earlier if breadcrumbs brown too quickly.) Serve warm.