Melt 6 tablespoons of butter in the microwave in a microwave-safe bowl and set aside to cool. Melt the remaining 2 tablespoons of butter in a 10-inch cast-iron skillet in the oven while it preheats to 400°F.
Whisk the mashed sweet potato, buttermilk, cooled melted butter, brown sugar, molasses, vanilla, and eggs in large mixing bowl until combined.
Add the cornmeal, flour, baking powder, salt, cinnamon, nutmeg, cloves, and cardamom and fold into the wet ingredients until no more dry flour is visible. The batter will be thick.
Carefully remove the skillet from the oven and spoon the batter into the hot skillet and smooth it out into an even layer. Bake the cornbread for 20-25 minutes or until the edges are golden brown and a toothpick inserted into the center comes out clean.
Meanwhile, whisk the melted butter, maple syrup, and salt in a small bowl until smooth. Brush the maple butter over the hot cornbread. Allow the cornbread to cool for about 10 to 15 minutes before slicing.