Combine the softened butter, maple syrup, and sea salt in a small bowl. Beat with a hand mixer at medium speed until smooth, about 1 minutes. Cover and chill. Soften at room temperature to serve.

In the 2-quart pot, put the potato pieces in ½-inch of water and bring to a boil. Reduce heat to medium and continue to cook until fork tender, about 10 more minutes. Drain and allow the potato to cool to room temperature. Mash well.

Preheat the oven to 350° F/175° C. Soak the dried cranberries in a bowl of hot water until soft.

Whisk together the whole wheat pastry flour, unbleached all-purpose flour, baking soda, baking powder, salt, cinnamon, and allspice in a the 3 quart mixing bowl; make a well in center of mixture.

In the bowl of the stand mixer using the paddle attachment, mix sugar, 1 cup of the mashed sweet potato, melted butter, and eggs on low speed. If the mixture is very wet and the butter doesn't mix in well, add another tablespoon or two of sweet potato and mix well. Pour the wet mixture into the dry ingredients, stirring with a spatula just until moistened, about 15 turns. Drain the soaked cranberries and fold in with the muffin mixture. Scoop the batter into the greased muffin pan, filling each cup about ⅔ full.

Bake in the preheated oven until the muffins are golden brown, about 20 to 23 minutes. Remove from the pan immediately, and cool on wire racks for a few minutes, then serve with the Maple Butter.