Preheat the oven to 400°F. Thoroughly grease a 24-count mini muffin tin.

Using a hand mixer or stand mixer, beat the vegetable oil and sugars until smooth. Add the egg, beating to combine. Stir in the baking powder, baking soda, cinnamon, salt, and vanilla. Add half the flour, beat until combined. Add the milk, beat until combined. Add the remaining flour, beat until combined.

Divide the batter between 24 mini muffin cups. I used a cookie scoop and placed a heaping 2 teaspoons of batter into each cup. Bake for 8-10 minutes, until the top springs back when you touch it. Place on a rack to cool for a few minutes.

Set up two large shallow bowls. Place melted butter in one, and combine sugar and cinnamon in the other. Once the donut holes are cool enough to handle, lightly brush them with butter, then immediately roll in cinnamon sugar.

Fill a piping bag fitted with a small open star tip with Nutella and gently insert the tip into the side of each donut hole and squeeze a little Nutella inside.