Place flour, salt, sugar, milk, and eggs in a blender. Mix on high until completely combined, about 30 seconds. Let mixture rest in the refrigerator for at least 30 minutes or overnight. (If the batter separates, mix it again just before cooking the crêpes.)
When ready to cook the crêpes, melt 1/2 tablespoon of the butter in a 10-inch nonstick skillet or an 8-inch crêpe pan over medium heat. As the butter melts, swirl it around to coat the bottom of the pan. Pour in about 1/4 cup of the batter and immediately swirl and tilt the pan to create a thin, even layer. (If the batter sets before the skillet is coated, reduce the heat slightly. The next crêpe will be better.) Return to heat and cook until the crêpe is golden around the edges and dry in the c
Slide the crêpe onto a serving plate and fold it in half, then in half again. Repeat with the remaining batter, buttering the pan as needed (after about every other crêpe). Serve immediately.