Whisk flour, baking powder, and salt in a large bowl.
Whisk egg, sugar, milk, oil, and vanilla in another bowl until smooth.
Combine wet and dry mixes. Stir gently; a few lumps are fine.
Spray with nonstick spray and preheat for 3–5 minutes.
Add enough batter to cover plates without overflowing. Cook 5–7 minutes until golden.
Cool slightly, then cut each waffle in half horizontally.
Spread pizza sauce, sprinkle mozzarella, and top with pepperoni on bottom halves.
Broil 1–2 minutes until cheese is bubbly.
Add top halves, press gently, and serve hot.