To make the potatoes, wash outside of potatoes and pat dry with paper towels. Rub with salt and wrap in heavy-duty aluminum foil. Place on a baking sheet and cook until fork tender, about 45 minutes. Remove from oven and allow to cool. Unwrap potatoes and cut in half. Carefully remove flesh from skin and place in a bowl and mash.
To make the caramel, in a large saucepan over medium-low heat, add butter and cook until butter turns golden brown and light brown sediment forms on bottom. Whisk in sugar and buttermilk. Increase heat to medium. Whisk together until sugar dissolves. When butter mixture starts to boil, slowly whisk in baking soda, maple, and vanilla. The mixture will bubble up. Remove 1 1/4 cups syrup and place remainder in airtight container for later use.
To assemble, combine sweet potato flesh, cream, caramel, salt, and allspice. Transfer to an ovenproof serving dish and top with pecans and remaining syrup. Place in oven until reheated, 20-25 minutes.