1/4 cup cinnamon mixed with 1 cup sugar, for dipping
Chocolate Glaze, for dipping (optional)
In a large bowl, sift the flour with the baking powder, nutmeg, salt, baking soda and mace. In a small bowl, whisk the eggs and egg yolk with the sugar and the melted butter. Stir in the buttermilk and molasses. Add the liquid mixture to the dry ingredients and mix vigorously with a wooden spoon, until a soft and slightly sticky dough forms.
Turn the dough out onto a heavily floured surface and gently knead 3 times. Roll out the dough to a 10-by-14-inch rectangle 1/4 inch thick. Stamp out the doughnuts with a floured 3-inch doughnut cutter (see Note), dipping the cutter in flour between each cut. Let the doughnuts rise for 5 minutes before frying. Meanwhile, gently knead and reroll any remaining dough and repeat.
In a large, heavy saucepan or deep cast-iron skillet, heat the peanut oil until it registers 375° on a deep-frying thermometer. Working in small batches of 3 or 4, carefully slide 1 doughnut at a time into the hot oil; the temperature of the oil should drop to 360° as soon as you have a full batch. As soon as the doughnuts float to the surface, turn them with a slotted spoon and fry for 45 seconds. Turn them again and fry for 45 seconds longer. Carefully transfer the doughnuts to paper towel-lined racks and drain on both sides. Dip the warm doughnuts in the cinnamon sugar or warm Chocolate Glaze and serve at once.
A round biscuit cutter is a fine substitute for a doughnut cutter. Use an apple corer with a round edge to stamp out the doughnut hole.