Wash the lemons with a vegetable brush and hot water to remove any reside of pesticides or wax; pat the lemons dry.
Carefully zest the lemons with a zester or vegetable peeler so there is no white pith on the peel. NOTE: Use only the outer part of the rind. The pith, the white part underneath the rind, is too bitter and would spoil your limoncello.
In a large glass jar (1-gallon jar), add one bottle of vodka; add the lemon zest as it is zested. Cover the jar and let sit at room temperature for at least (10) ten days and up to (40) days in a cool dark place. The longer it rests, the better the taste will be. (There is no need to stir - all you have to do is wait.) As the limoncello sits, the vodka slowly take on the flavor and rich yellow color of the lemon zest.
In a large saucepan, combine the sugar and water; cook until thickened, approximately 5 to 7 minutes. Let the syrup cool before adding it to the Limoncello mixture. Add to the Limoncello mixture from Step One. Add the additional bottle of vodka. Allow to rest for another 10 to 40 days.
After the rest period, strain and bottle: discarding the lemon zest. Keep bottles you are using in the freezer until ready to serve.
Note: Use coffee filters for straining. If you moisten the filters before the straining, you will not waste liqueur by soaking it into the filter.
* Begin filtering Limoncello. Use punch ladle to pour a small amount into a filter-lined funnel held over the small measuring cup.
* Fill measuring cup to an even ml level. (100, 200, 300, etc.)
* Using smaller funnel and #2 sized filters, filter one last time into individual 100-ml bottles.
* Seal bottles VERY tightly. Remember, if you are using different bottles and are sealing them by corking -- corks breathe. So consider dipping the neck several times in melted wax after corking.
* Label and/or tag bottles.
* Return to cool dark place for storage.