In a large bowl whisk together the flour, salt, sugar, baking powder and zest.
Make a well in the center and add the butter, egg and rum, mix with a fork until almost combined.
Move to a lightly floured flat surface and knead into a soft smooth dough, don’t over knead. If the dough is too sticky then add a bit more flour, too dry then add a bit more butter.
Wrap the dough in plastic wrap and refrigerate for 30 minutes.
Remove the dough from the fridge and form into small balls about the size of small chestnuts.
In a medium pot add about 2 inches of vegetable oil and heat to 300-325F (150-162C), once the correct temperature is reached, depending on the size of your pot add the dough balls in batches, be sure not to crowd them.
Turn them often until they are golden brown all over, approximately 3 minutes in total. Remove the castagnole with a slotted spoon and place on a kitchen paper/paper towel lined plate to help drain excess oil. While still warm roll in sugar and serve! Enjoy.
Notes
The best oil to use are peanut, sunflower or canola oils. Be sure to heat the oil to a temperature of 300-325F (150-160C). Try to keep it as close to this temperature as possible.Lower than this and they will be soggy and not cooked, too high and they will cook too much on the outside and not enough on the inside.To bake the castagnole, place the formed balls on a parchment paper lined cookie sheet and bake in a pre-heated 350F (180C) oven for approximately 12-15 minutes. Instead of rolling in sugar, dust the cooled castagnole with icing/powdered sugar.This is the type of fried food that should be eaten immediately they day they are made, the longer they sit out the drier they will become. Although baked will keep in an airtight container for up to 3 days at room temperature.