Combine flour and salt in a large measuring cup with a spout (or alternatively, in a bowl) and pour in egg and milk. Whisk to combine and refrigerate one hour. Make pastry cream.
When ready to fry crêpes put a small skillet over medium heat. When hot add a small amount of butter and swirl the coat the skillet. Quickly pour in batter and swirl to coat the bottom of the pan. The first ones may not look perfect, but you can fold them up to look top-notch. As you get the hang of it, you'll get better! Cook until small blisters appear under the surface and the outer edges begin to curl away from the skillet. Gently flip and cook another 30 seconds or until golden. Remove to a plate and repeat with remaining batter, buttering the skillet as often as necessary to keep crêpes from sticking.
Pastry Cream
In a small saucepan combine milk, 1/4 cup sugar, cinnamon, lemon zest, and salt. Bring to a simmer. Meanwhile, in a medium bowl whisk together yolks, cornstarch, and remaining 1/4 cup sugar.
When milk gets to a simmer, slowly ladle about 1/2 cup of milk into yolk mixture, whisking constantly. In a slow stream, continue to pour in milk, whisking constantly. Return mixture to saucepan over medium heat and cook until thickened, about 3 minutes, stirring constantly.
Remove from heat and whisk in butter and vanilla. Cover with cellophane directly on top of the creme and refrigerate 2 hours or up to 2 days.
Assembly
Determine whether you'd like to serve your crêpes chilled, room-temperature, or warmed. You may reheat them on a greased cookie sheet in the oven set to 300 degrees Fahrenheit for about 12 minutes.
Place a tablespoon or two of lemon pastry cream into one quarter of the crêpe and fold it in half and half again. Alternatively, you can fill the crêpes right down the center and roll up like mini French burritos.