Preheat oven to 400ºF. Using a fork, poke the sweet potatoes all over to allow steam to escape while they bake, and place them in the oven. Bake for 45 to 55 minutes or until the sweet potatoes are fork tender. Remove from the oven and add to a large bowl, carefully peeling away the skins. Add the butter and milk and mash well. Add the brown sugar, cinnamon, nutmeg, allspice, ginger, vanilla and a good pinch of salt. Mash some more to make them super creamy.