Sift flour and salt into a large bowl. Combine 1 1/4 cups milk, egg, and oil in a second bowl and beat until well combined. Add milk mixture to flour mixture and stir to combine. Set batter aside for 30 minutes.
Melt 1 teaspoon butter in an 8-inch skillet over medium heat. Pour a small ladleful of batter into the skillet. Immediately rotate the skillet until batter coats the cooking surface in a thin layer. Cook until the top of the crepe is no longer wet and the bottom has turned light brown, 1 to 2 minutes. Run a spatula around the edge of the skillet to loosen crepe; flip crepe and cook until the other side has turned light brown, about 1 minute more. Slide crepe onto a warmed plate. Repeat with remaining batter.
Melt 2 teaspoons butter in a saucepan; stir in 1 tablespoon milk and hazelnut liqueur. Add confectioners' sugar and cocoa powder and keep warm over very low heat.
Distribute crepes amongst plates. Add 1/4 of the banana slices onto each crepe and pour 1/4 of the chocolate sauce on top. Roll up crepe and fold. Dust with confectioners' sugar.