In a large bowl add the flour and the mashed potatoes. Dissolve the yeast in warm water, add a pinch of salt and add to the bowl. Mix all the ingredients well and work the dough for about 5 minutes. Alternatively you can use a stand mixer. The consistency of the dough should be rather soft. Add a bit of extra water or flour to adjust. Cover the bowl with a cloth and leave to rise in a warm environment for about 1 hour. Get your oil very hot in a deep frying pan and with a spoon drop strands of dough in irregular shapes. Turn the dough in the oil until it is golden in color. Remove the Zeppole from the oil and quickly sprinkle with a generous dose of granulated sugar on all sides. Alternatively you can use powder sugar.