2In a large bowl, sift together flour, 1 cup sugar, baking powder, salt, nutmeg, and baking soda.
In a medium bowl, whisk together buttermilk, eggs, and egg yolk. Add buttermilk mixture to flour mixture, stirring well. Refrigerate for 10 minutes. Place dough on a floured surface. Using floured hands, Pat to 1/2 inch thickness. Using a doughnut cutter, cut into 12 doughnuts, re- rolling scraps as needed. Place on prepared sheet.
3Line another baking sheet with paper towels.In a small bowl, mix remaining 1 cup sugar and cinnamon.
4In a large heavy bottomed Dutch oven or cast iron skillet, pour vegetable oil to a depth of 4 inches. Heat oil over medium-high heat to 375°. Adjust heat as needed to maintain temperature, using a candy thermometer to regulate. Fry doughnuts, 1 to 2 at a time, being careful not to overcrowd pan or allow temperature to drop. Cook for 1 1/2 to 2 minutes per side, or until Golden Brown. Remove from oil, and place on sheet lined with paper towels to drain. Repeat with remaining doughnuts.
Sprinkle with cinnamon sugar while still hot. Serve at once. Makes about one dozen doughnuts