01 - Place the potatoes in a large pot and cover with cold, salted water. Bring to a boil over high heat, then reduce to a simmer. Cook until potatoes are fork-tender, about 15 to 20 minutes.
02 - Melt the butter in a small saucepan over medium heat. Add the sage leaves. Cook, swirling occasionally, until the butter turns golden brown and sage is crisp, about 3 to 4 minutes. Remove from heat immediately to prevent burning.
03 - Drain the potatoes thoroughly and return them to the pot. Allow them to sit for 1 to 2 minutes so excess moisture evaporates.
04 - Mash the potatoes using a potato masher or ricer until they reach a smooth consistency.
05 - Gently heat the milk and cream (if using) until warm, then pour over the mashed potatoes.
06 - Add the brown butter and crispy sage to the potatoes, reserving a small portion for garnish. Season with salt and freshly ground black pepper. Mix until creamy and thoroughly combined.
07 - Taste and adjust seasoning as needed. Transfer to a serving bowl and finish with a drizzle of reserved brown butter and sage.