In a large, microwave safe bowl, melt the butter. Mix in the maple syrup, vanilla, water and egg yolk (reserve the egg white in a separate bowl) until well combined. *Ensure the other ingredients are at room temperature so that the butter doesn't solidify.
Whisk in the oat flour, salt and baking powder until just combined.
In a separate bowl, use a hand blender to whip the egg white until stiff peaks form. Fold it into the batter until just combined.
Allow the batter to rest for 5-10 minutes while you preheat a waffle iron.
Spoon ~¼-1/3 cup of batter (be careful not to overfill) onto your heated waffle iron and cook until lightly browned and crisp (3-5 minutes). Repeat with remaining batter.
Enjoy!
Notes
LeftoversAllow leftover oat flour waffles to cool completely on a wire rack. Once cool, store leftover waffles in an airtight container in the refrigerator for up to 3 days or freeze for up to 3 months. To rewarm, pop them in the toaster or reheat in the microwave.