Put a large frying pan on the heat and melt the butter over low heat. Cut the onion into very small pieces and then brown it in the butter for a couple of minutes. When the onion has browned, add the rice to the pan and toast it for a couple of minutes; then add the white wine and let the rice evaporate. Pour the vegetable broth into the pan a little at a time, adding it as the rice absorbs it and adjust the salt to your liking; turn the rice several times with a wooden spoon so that it cooks evenly and does not stick to the pan. Add the saffron powder when the rice is about halfway through cooking and mix everything well with a wooden spoon. When the rice is about 3/4 cooked, add the Parmesan cheese and a further knob of butter. Finish cooking the saffron risotto and serve it still hot.