In a small bowl, mix flour and salt; set aside.

In a large bowl, beat egg whites with an electric mixer on high speed until stiff peaks form.

In another large bowl, beat egg yolks with an electric mixer on high speed until thickened and lemon-colored, about 5 minutes. Stir in melted butter and vanilla. Alternately add flour mixture and milk to yolk mixture, beating well after each addition. Fold egg whites into yolk mixture.

Prepare Belgian waffle iron according to manufacturer's directions. Bake on waffle iron until steaming stops and waffle is golden brown.