Preheat a square waffle iron.Whisk flours with baking powder, sugar and salt in a medium bowl.Whisk egg and yolk with milk in a large bowl. Add flour mixture to egg mixture while stirring, until almost combined.Stir in butter until smooth. Be careful not to overmix. Ladle mixture into waffle iron and cook according to manufacturer’s directions. You should have 8 waffles.Heat a large non-stick pan over medium. Arrange bacon slices in pan. Cook, flipping halfway through, until crisp, 8 to 10 min. Transfer bacon to a paper towel-lined plate.Assemble waffle-wich by brushing mayo on 4 waffles, then layering with bacon, lettuce and tomato. Sandwich with remaining waffles.