01 - Bring salted water to a boil. Add elbow macaroni and cook until al dente, about 7-8 minutes. Drain and set aside.
02 - Melt 2 tablespoons butter in a saucepan over medium heat. Whisk in 2 tablespoons flour and cook for 1 minute. Gradually add whole milk while whisking constantly. Cook for 2-3 minutes until sauce thickens slightly.
03 - Stir in shredded cheddar and mozzarella until melted and smooth. Season with salt, black pepper, and ground mustard. Fold in cooked macaroni. Let the mixture cool slightly.
04 - In a bowl, whisk together flour, sugar, baking powder, baking soda, and salt. In a separate bowl, whisk buttermilk, eggs, and melted butter. Pour wet ingredients into dry and mix until just combined.
05 - Preheat waffle iron to medium-high and lightly grease with nonstick spray or melted butter.
06 - Spoon a thin layer of waffle batter to cover the bottom of the waffle iron. Place a scoop of mac and cheese in the center, leaving a border. Cover with more batter. Close waffle iron and cook for 4-6 minutes until golden and crisp.
07 - Repeat with remaining batter and mac and cheese. Serve waffles immediately while hot.