Preheat the oven to 425 degrees. Butter a large cast iron skillet, or baking sheet.
Cook the sweet potatoes until they are soft. Add two tablespoons of the milk, and mash them up until they are smooth. Measure out one cup, set them aside to cool.
While the sweet potatoes are cooling, add the flour, baking powder, cinnamon sugar mixture, and salt to a stand mixer. Use the whisk attachment and mix on low for about a minute, until everything is just combined, (or use a whisk or electric hand mixer).
Use a box grater to grate the ice cold butter, add it into the flour mixture. Use a pastry cutter to cut it into the mixture until it is combined (or use your hands if you do not own a pastry cutter). It will be "meal like" with pieces of butter throughout.
Transfer the mixture to a large bowl, and add the mashed sweet potatoes. Then add in the rest of the milk, and gently fold it all together until it is combined and a soft/semi sticky dough is formed.
Sprinkle some flour out onto a clean work surface, and transfer the dough out onto it. Work the dough with your hands, flattening it out to about 1/2 inch thick, and folding it over onto itself. Do this 3 times.
Then pat the dough out flat, about 3/4" thick. Use a 2 1/2 inch biscuit cutter, or glass to cut out the biscuits out of the dough. Gather the scraps together and pat them out flat to cut out the remaining dough.
Add the biscuits to the buttered cast iron skillet or baking sheet. Bake for 12-15 minutes, until they are golden brown and cooked through.
Serve with cinnamon sugar butter.