For the batter, whisk together the eggs, water, rum, vanilla, sugar and salt in a large bowl, until well blended.
Sift the flour, a bit at a time, whisking each addition until smooth.
Add in half the melted butter and the lemon zest, whisking until the batter has slightly thickened, with the consistency of melted ice cream. Put the remaining butter in a small cup and keep it warm.
Heat the crepe pan or a nonstick skillet over medium heat until quite hot. Lightly brush with melted butter. Ladle in a scant ? cup of batter, tilt and swirl so it coats the bottom. Lower the heat and cook the pancake until underside is golden brown, 2 to 3 minutes. Flip it over with a spatula and fry for a half minute or longer, until the second side is lightly browned. Slide the crepe out of skillet and repeat with the remaining batter. (Coat pan with butter as needed.).
Fold the creps and squeeze some lemon juice over them, sprinkle with granulated sugar or cinnamon or spread with a filling of your choice.