Make the doughnut holes: Whisk the flour, cocoa powder, baking powder, salt and cinnamon in a medium bowl. Beat the butter and sugar in a large bowl with a mixer on medium-high speed until smooth, about 3 minutes. Beat in the eggs, one at a time, then beat in the vanilla and orange zest until light and fluffy, about 2 more minutes. Reduce the speed to low and gradually beat in the flour mixture, alternating with the sour cream, until a sticky dough forms, about 2 minutes. Cover the dough with plastic wrap; refrigerate at least 3 hours or overnight.
Coat a baking sheet with cooking spray. Heat 2 inches of vegetable oil in a large pot over medium-high heat until a deep-fry thermometer registers 340 degrees F. Meanwhile, roll out the dough until 1/2 inch thick on a floured surface. (The dough will be sticky.) Cut out rounds using a 1-inch-round cookie cutter, dipping the cutter in flour each time. Place the rounds on the prepared baking sheet.
Set a rack on another baking sheet. Working in batches, carefully fry the doughnut holes until puffed and browned, 30 seconds per side. Transfer to the rack using a slotted spoon or wire strainer.
Make the glaze: Line a baking sheet with parchment paper. Put the chocolate in a small heatproof bowl. Bring the heavy cream to a boil in a small saucepan and immediately pour over the chocolate; cover and let stand 5 minutes. Whisk until the chocolate is melted and smooth, then whisk in the corn syrup and vanilla. Set aside until cooled and thickened, about 5 minutes. Dip the doughnut holes in the glaze, transfer to the prepared baking sheet and sprinkle with nonpareils. Let sit at least 1 hour before serving.