Don't worry if all your crepes don't turn out perfectly; there's enough batter to allow for a few mistakes.
To prepare crepes, lightly spoon flour into a dry measuring cup; level with a knife. Place flour, sugar, and salt in a medium bowl; stir with a whisk. Combine 1 1/2 cups milk and eggs, stirring with a whisk. Add milk mixture to flour mixture, stirring with a whisk just until smooth. Cover batter; chill for 15 minutes.
Heat a 10-inch 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. 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.
Place crepe on a towel; keep warm. Repeat procedure until all the batter is used. Stack crepes between single layers of wax paper or paper towels to prevent sticking.
To prepare sauce, combine the hazelnut-chocolate spread, 2 tablespoons milk, and vanilla in a small saucepan over medium heat, stirring with a whisk until smooth. Keep warm.
Peel bananas and cut in half lengthwise; cut each half crosswise into 2 pieces.
Heat a large nonstick skillet coated with cooking spray over medium-high heat. Arrange 4 banana pieces in a single layer in pan. Cook 1 minute or until lightly browned. Turn pieces over; cook 1 minute. Remove banana pieces from pan; keep warm. Repeat procedure with remaining banana pieces.
Place 1 banana piece in center of each crepe; fold sides and ends over, and place, seam side down, on clean surface. Repeat procedure with remaining banana pieces and crepes.
Spoon about 1 tablespoon sauce onto each of 4 plates, spreading to cover center of plates. Arrange 2 crepes on each plate; sprinkle with powdered sugar, if desired. Serve immediately.