The simplest way to fill a donut is to use a piping bag. Cut the tip off of the piping bag and insert your filling of choice. It is easiest to have a piping tip that has a large opening. The tip will help make sure the filling gets into the middle of the doughnut. Pipe the Nutella filling into the center of the doughnut. Depending on the size and density of the doughnut, you might need to squeeze the filling in from each size. Another option is to cut the doughnut in half and fill it like a sandwich or spread the Nutella over the top of the fried doughnut.