Preheat oven to 176C | 350F. Spray a 24-count mini muffin tray with cooking oil spray; wipe over excess with paper towel and set aside.
In a large bowl, whisk all of the dry ingredients together.
Make a well in the center and add the oil, egg, vanilla and milk.
Whisk the batter until smooth and lump free.
Using a small ice cream scoop or spoon 2 tablespoons into each muffin tin, filling to to ¾ full. Bake for 10-15 minutes, or until they are golden in color and a toothpick inserted into the center of a donut comes out clean.
Allow them to cool for 5 minutes and transfer them onto a cooling rack.
Brush each donut over with a light coating of melted butter; combine the sugar and cinnamon together in a shallow bowl; toss each donut hole into the cinnamon sugar mix until evenly coated.
Fill a piping bag with the Nutella. Soften it about 10-15 seconds in microwave if it's not softened enough to pipe through to fill donut.
Using a a narrow nozzle, fill each donut with Nutella until they puff up and expand.