Preheat your oven to 400°F (200°C). Place whole sweet potatoes on a baking sheet lined with parchment paper and roast them for 45-55 minutes, until tender when pierced with a fork.

In a medium saucepan over medium heat, melt ½ cup of salted butter until golden brown, stirring frequently for about 4-6 minutes.

In a large mixing bowl, combine the mashed sweet potatoes with the browned butter, dark brown sugar, granulated sugar, heavy whipping cream, a large egg, and egg yolk, vanilla extract, lemon juice, cinnamon, nutmeg, and a pinch of salt. Mix until fluffy and well-blended.

Spread the sweet potato mixture into a greased 7x10 or 8x8 baking dish. Preheat your oven to 350°F (175°C) and bake for 40-45 minutes.

Remove the casserole from the oven and sprinkle a light layer of dark brown sugar, then top with mini marshmallows or marshmallow fluff. Return to the oven for 8-10 minutes or broil for 1-2 minutes until marshmallows are golden.

Allow to rest for about 10 minutes before serving to let the flavors meld.