01 - Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl. In a separate bowl, combine milk, eggs, melted butter, and vanilla extract. Pour wet ingredients into dry mixture and stir until just combined. Avoid overmixing to ensure tender waffles.
02 - Preheat waffle iron and lightly grease with cooking spray or oil. Pour batter onto iron and cook according to manufacturer instructions until golden brown and crisp. Prepare 8 waffles total (4 burgers require 2 waffles each). Keep cooked waffles warm in a low oven.
03 - Preheat oven to 400°F. Arrange bacon slices on a foil-lined baking sheet. Bake for 12-15 minutes until partially cooked, then remove and brush both sides with maple syrup. Return to oven for 3-4 minutes until caramelized and crispy. Let cool briefly, then cut each slice in half.
04 - Divide ground beef into 4 equal portions and shape into patties slightly larger than the waffles (they will shrink during cooking). Season both sides generously with salt and pepper.
05 - Heat skillet or grill pan over medium-high heat. Cook patties for 3-4 minutes per side for medium doneness. Place cheddar cheese slice on each patty during the final minute of cooking and cover to melt.
06 - Whisk together mayonnaise, maple syrup, Dijon mustard, and pinch of salt in a small bowl until smooth and well combined.
07 - Butter one side of each waffle. Place buttered side down in a skillet over medium heat and toast until golden brown and crispy, about 1-2 minutes.
08 - Spread maple aioli on the untoasted side of a waffle. Layer with arugula, cheese-topped burger patty, maple bacon halves, and red onion slices. Top with second waffle, untoasted side facing inward. Repeat for remaining burgers.
09 - Serve immediately while waffles remain crisp and burgers are hot. Cut in half if desired for easier eating.