Preheat oven to 180ºC . Peel and boil sweet potatoes in a covered saucepan until tender about 20 min. Drain and mash. Mix in the softened butter, sugar, vanilla, and milk. Pour into a baking dish or casserole. Bring cream to simmer in a small saucepan. Add brown sugar and stir until dissolved. Cook over medium heat to soft ball stage 110ºC . Remove from heat and beat in butter and pecans. Pour over sweet potato mixture. All of the above may be done up to 2 days in advance and stored in the fridge. Then just heat in the oven until bubbling hot and enjoy.