Preheat the oven to 425℉ and line a large baking sheet with parchment paper.
Place cubed sweet potatoes on baking sheet, drizzle with olive oil and toss to coat; sprinkle with salt and pepper. Roast at 425℉ for about 30 minutes, stirring every 10 minutes, until soft and slightly caramelized. Remove from oven, carefully transfer to a large mixing bowl and mash thoroughly. Transfer to casserole dish and set aside.
Reduce oven temperature to 400℉. Place butter and brown sugar in a heavy bottom saucepan, cook over medium heat until melted. Whisk to combine and bring to a boil, whisking often. Boil over medium heat for about a minute, while constantly stirring. Remove from heat and pour over mashed sweet potatoes.
Bake at 400℉ for 20 to 30 minutes, until topping is bubbly, and starting to caramelize. Watch carefully after 20 minutes to make sure the topping does not burn. Remove from oven and cool for just a minute before serving.
Notes
Store leftovers in an airtight container in the refrigerator for up to 2 days.