To prepare crepes, lightly spoon the flour into dry measuring cups, and level with a knife. Place flour, milk, 1 teaspoon sugar, and egg in a blender. Process until smooth. Cover and chill 30 minutes.
Heat an 8-inch crepe pan or nonstick skillet coated with cooking spray over medium-high heat. Remove pan from heat. Pour a scant 1/4 cup batter into pan; quickly tilt pan in all directions so batter covers pan with a thin film. Cook about 1 minute.
Carefully lift edge of crepe with a spatula to test for doneness. The crepe is ready to turn when it can be shaken loose from the pan and the underside is lightly browned. Turn crepe over; cook 30 seconds on other side.
Place crepe on a towel; cool. Repeat procedure until all of the batter is used. Stack the crepes between single layers of wax paper or paper towels to prevent them from sticking.
Place 1 crepe, browned side down, on a plate. Spoon about 1 1/2 tablespoons Dulce de Leche down center of crepe. Roll up crepe, and place on a heatproof platter. Repeat procedure with remaining crepes and Dulce de Leche. Brush butter over tops of crepes; sprinkle with sugar. Broil crepes 2 minutes or until lightly browned.