Set your oven at 450° to preheat.
Combine the dry ingredients in a large mixing bowl. Break up any "lumps" in the dry goods.
Add the sweet potatoes and butter to the flour mixture. Use the pastry tool to mix all into a soft dough. Depending on the "softness" of your dough, you can add the milk if it is dry. Normally, about two tablespoons of milk is all you will need. This largely depends on how "wet" your potatoes are.
Form the dough into a ball. Turn it onto a pre-prepared silicone baking mat, lightly coated with flour.
Roll the dough into a 1/2" thickness and cut them with the biscuit cutter. As you cut, you can "rework" the scraps into the mixture.
Put each biscuit onto greased baking sheet or an ungreased stoneware sheet.
Place the baking sheet into the oven. Bake for 15 minutes, or until the bottoms become slightly golden brown.
Remove and cool on a platter.