Fit a large heavy pot with a deep-fry thermometer. Heat 2 inches of oil to 350 degrees F.
Place the rice flour and baking powder in a large heatproof bowl, then whisk to combine and set aside. Combine the brown sugar, salt and 1 cup water in a medium saucepan and bring to a boil, stirring so that the sugar dissolves. Remove it from the heat and stir it into the rice flour to form a thick, sticky, doughy batter. Let it cool slightly, for about 5 minutes, then turn it onto a surface liberally dusted with rice flour. Knead it, adding more flour as needed, until it is just smooth enough to handle (this may take quite a bit more flour, up to 1/3 cup), for about 2 minutes. It should still be slightly sticky. Divide it into 12 balls and cover them with plastic wrap.
Place the sesame seeds in a small bowl or on a plate. To shape, flatten each ball slightly with your thumb, add about 3/4 teaspoon chocolate hazelnut spread and then gather up the sides and pinch to seal shut. Roll it in the sesame seeds to coat it all over, brushing with a bit of water if it's too dry for the seeds to stick.
Fry the balls in batches until they're browned, about 5 minutes. Remove with a slotted spoon and transfer to a plate lined with a paper towel. Let cool slightly and serve. These are best enjoyed immediately.