What’s better than one big doughnut? A whole bunch of mini doughnut holes! (Get it?!)
Doughnuts
Topping
For the doughnuts: Adjust oven rack to middle position and heat oven to 400 degrees. Spray 24-cup mini muffin tin well with vegetable oil spray.
In medium bowl, whisk together flour, cornstarch, baking powder, salt, nutmeg, and ½ cup sugar.
In large bowl, whisk buttermilk, 4 tablespoons melted butter, egg, and egg yolk until well combined.
Add flour mixture and use rubber spatula to stir until just combined and no dry flour is visible. Do not overmix.
Spray 1-tablespoon measuring spoon with vegetable oil spray. Use greased measuring spoon to scoop 1 tablespoon batter into each muffin tin cup.
Place muffin tin in oven. Bake until toothpick inserted in center of 1 doughnut hole comes out clean, about 10 minutes.
Use oven mitts to remove muffin tin from oven (ask an adult for help). Place muffin tin on cooling rack and let doughnut holes cool in muffin tin for 10 minutes.
For the topping: In small microwave-safe bowl, melt remaining 4 tablespoons butter. In second medium bowl, use clean rubber spatula to stir together cinnamon and remaining ½ cup sugar.
Carefully remove doughnut holes from muffin tin and transfer directly to cooling rack (ask an adult for help—muffin tin will be hot). Place cooling rack in rimmed baking sheet. Use pastry brush to paint doughnut holes all over with melted butter (use all of butter). Then, roll each doughnut hole in cinnamon sugar to coat all over. Serve.