In a large bowl, whisk flour, sugar, baking powder (or yeast), and salt. In another bowl, whisk egg yolks, milk, melted butter, and vanilla. Combine wet and dry ingredients gently.
Beat egg whites until stiff peaks form, then fold into the batter to keep it airy.
Preheat the waffle iron until fully heated for even cooking.
Pour batter into the iron, close, and cook for 4-5 minutes until golden brown.
Serve immediately dusted with powdered sugar, drizzled with syrup, or topped with fruit, whipped cream, or Nutella.