Beat the eggs and add the sugar, and continue beating until light and fluffy.
Add the liqueur (if using), milk, salt and lemon peel and continue to beat the mixture.
Add the flour, a little at a time, until the crepe batter is uniform and there aren't any lumps.
Optionally, you can let the batter rest for an hour, and then strain it to make sure there aren't any lumps.
Using a paper towel, coat a frying pan with olive oil (just a thin layer).
Add a very small amount of batter to the hot pan (starting with just two tablespoons). Spread it out to completely cover the pan.
After about 30 seconds, it should be ready to flip.
When each frisuelo is cooked, put in on a large plate and sprinkle sugar on top. Layer the frisuelos on top of one another, until you've used up all of the batter!