1. Heat the oven to 325°. Butter an 8-inch round metal cake or pie pan.
2. In a medium bowl, whisk together the flour and 3/4 teaspoon of the nutmeg. With your fingers, rub in the butter completely until the mixture is the texture of sand. Using a fork, stir in 5 tablespoons of the sugar. Stir in the egg yolk. Press the mixture together to make a dry, crumbly dough and put it on a work surface. Knead the dough about twelve times until it just holds together.
3. Press the dough in an even layer into the prepared pan. With a small, sharp knife, mark eight wedges halfway into the dough. With a fork, prick the dough every 1/2 inch or so. In a small bowl, combine the remaining 1 tablespoon sugar and 1/4 teaspoon nutmeg and sprinkle it on the dough. Bake the shortbread in the middle of the oven until golden, 50 to 55 minutes.
4. Let the shortbread cool slightly in the pan set on a rack and then cut into wedges. Let cool completely in the pan.