Sprinkle the yeast in a bowl containing 1/2 cup of lukewarm water and let this sit for a 10 minutes, till frothy, then stir to dissolve.
Add in 1 cup of the flour, a pinch of salt and stir till you get a soft batter. Cover the bowl with cling wrap and let this sit for about 30 minutes.
Next, stir in the remaining flour, sugar, the lightly beaten eggs, lemon zest, butter and vanilla extract. Mix well.
If kneading by hand: Transfer the dough to a work surface and knead for about 6 minutes. The dough should be soft, smooth and just a little bit sticky.
If using a stand mixer: Knead the dough in the bowl of a stand mixer with the dough hook attachment for about 3 minutes. When ready the dough will pull away from the sides of the bowl and feel somewhat tacky.
Lightly grease a large bowl and place the dough in the bowl.
Cover with cling wrap and let the dough rise in a warm draft free place for 1 to 1 ½ hours. The dough will double in volume.
When ready transfer the dough to a work surface, lightly punch it down with your hands and knead it for about 1 minute.
Divide the dough into 4 equal pieces.
Keep the dough covered with a clean dishcloth while working with 1 piece of dough at a time. (this will keep the dough from drying out)
Roll each piece of dough out into approximately an 18 inch long rope. (keep covered with clean dish cloth)
Cut each rope into one inch pieces and roll each piece of dough in the palm of your hands for rounder shapes.
Place the dough balls onto a lightly floured baking sheet. (keep covered)
Pour 2 inches of oil in a medium sized sauce pan or dutch oven and heat to 375* degrees. (use a deep frying thermometer for accuracy)
When the oil has reached 375 degrees F, carefully drop in about 6 pieces of dough at a time. (This will also depend on the size of your sauce pan). Don't over crowd them.
When the fritters are golden on one side, flip over to the other side.
Once golden brown on both sides, use a slotted spoon and drain the castagnole on paper towels.
Roll the castagnole in granulated sugar turning to coat all over.
Serve while still warm, piled high on a serving platter.
For a festive Carnival appeal you can drizzle them with warm melted chocolate and top with sprinkles.