Preheat your oven to 350°F. Line two muffin tins with approximately 16 paper liners and set them aside. Meanwhile, in a large bowl, prepare the cake mix following the instructions on the package. Add the instant vanilla pudding mix and ground cinnamon to the batter and stir until well combined. Divide the batter evenly among the muffin liners, filling each about three-quarters full. Bake for around 15 to 18 minutes, or until a toothpick inserted near the center comes out clean or with a few moist crumbs. Allow the cupcakes to cool completely. In the bowl of a stand mixer, beat the softened butter, vanilla extract, and cinnamon together until smooth and creamy. Gradually add the powdered sugar, about one cup at a time, mixing until the frosting is thick and fluffy. Add milk as needed to achieve the desired consistency. Pipe the frosting generously onto the cooled cupcakes, then top each with a freshly baked churro piece and a light sprinkle of cinnamon sugar.