In a blender or food processor, blend the eggs, milk, water, flour, sugar, vanilla, optional flavoring, and the 2 tablespoons melted butter for 5 seconds, or until smooth. Stir down and repeat, if necessary. Or, to mix by hand, sift the flour into a medium bowl and add the sugar. Whisk the eggs until blended, mix in the milk, water, vanilla, and optional flavoring, and whisk this mixture into the flour; stir in the 2 tablespoons melted butter. Cover and refrigerate for at least 1 hour (2 hours is preferable) or up to 24 hours.
Gently stir the batter if it has separated. Heat a seasoned 6- or 7-inch nonstick crêpe pan over medium-high heat until hot. (Use a 9- or 10-inch pan for larger crêpes.) Wipe the pan lightly with butter, lift the pan from the heat, and pour in 2 to 3 tablespoons of batter for a 6- or 7-inch pan, or about ¼ cup for a 9- or 10-inch pan, tilting and rotating the pan to coat the surface. Cook until almost dry on top and lightly browned on the edges, about 1 minute. Loosen the edges with ametal spatula and flip the crêpe over using your fingers or the spatula, then cook the other side for about 15 seconds, or until lightly browned. Turn the crêpe out onto a clean tea towel to cool. Repeat with the remaining batter, wiping the pan with butter as needed and stacking the crêpes as they are cooked.
For serving immediately, cover the crêpes with aluminum foil and keep them warm in a preheated 200°F oven. For serving later, wrap them in plastic wrap in quantities intended for each use and slip them in a self-sealing plastic bag. Refrigerate crêpes for up to 3 days, or freeze them for up to 2 months.