Preheat oven to 450 degrees F.
Select the baking pan by determining if a soft or crisp exterior is desired. For a soft exterior, use an 8- or 9-inch cake pan, pizza pan, or ovenproof skillet where the biscuits will nestle together snugly, creating the soft exterior while baking. For a crisp exterior, select a baking sheet or other baking pan where the biscuits can be placed wider apart, allowing air to circulate and creating a crisper exterior, and brush the pan with butter.
Fork-sift or whisk 2 cups of flour, cinnamon, and nutmeg in a large bowl, preferably wider than it is deeper, and set aside the remaining 1/4 cup of flour. Scatter lard over the flour and work in by rubbing fingers with the lard and flour as if snapping thumb and fingers together (or use two forks or knives, or a pastry cutter) until the mixture looks like well-crumbled feta cheese, with no piece larger than a pea. Shake the bowl occasionally to allow the larger pieces of fat to bounce to the top of the flour, revealing the largest lumps that still need rubbing. If this method took longer than 5 minutes, place the bowl in the refrigerator for 5 minutes to rechill the fat.
Make a deep hollow in the center of the flour with the back of your hand. Scoop the sweet potatoes into the hollow and stir with a rubber spatula or large metal spoon, using broad circular strokes to quickly pull the flour into the sweet potatoes. Mix just until the dry ingredients are moistened and the sticky dough begins to pull away from the sides of the bowl. If too dry, add 1 to 4 tablespoons of milk.
Lightly sprinkle a board or other clean surface with some of the reserved flour. Turn the dough out onto the board and sprinkle the top lightly with flour. With floured hands, fold the dough in half, and pat dough out into a 1/3- to 1/2-inch-thick round, using a little additional flour only if needed. Flour again if necessary, and fold the dough in half a second time. If the dough is still clumpy, pat and fold a third time. Pat dough out into a 1/2-inch-thick round for a normal biscuit, 3/4-inch-thick for a tall biscuit, and 1-inch-thick for a giant biscuit. Brush off any visible flour from the top. For each biscuit, dip a 2-inch biscuit cutter into the reserved flour and cut out the biscuits, starting at the outside edge and cutting very close together, being careful not to twist the cutter. The scraps may be combined to make additional biscuits, although these scraps make tougher biscuits.
Using a metal spatula if necessary, move the biscuits to the pan or baking sheet. Bake the biscuits on the top rack of the oven for a total of 12 to 14 minutes, depending on thickness, until light golden brown. After 6 minutes, rotate the pan in the oven so that the front of the pan is now turned to the back, and check to see if the bottoms are browning too quickly. If so, slide another baking pan underneath to add insulation and retard browning. Continue baking another 6 to 8 minutes until the biscuits are light golden brown.
Meanwhile, whisk the confectioners' sugar and milk until smooth to make an icing. When the biscuits are done, remove from oven and slide them onto a rack over a piece of wax paper. Drizzle the icing over the warm biscuits. Discard the paper with the excess icing. Serve hot right away.
Sift together 1 cup all-purpose flour (or 1/2 cup cake flour with 1/2 cup all-purpose flour), 1/2 to 1 teaspoon salt, and 1 1/2 teaspoons baking powder.
Variation: To make a savory biscuit, omit the optional spices. For bacon biscuits, add 1/2 cup crumbled cooked bacon before adding the sweet potatoes.
Recipe from Southern Biscuits by Nathalie Dupree and Cynthia Graubart. Reprinted with permission by Gibbs Smith.