Preheat a waffle iron according to manufacturer's instructions. Grease with cooking spray.
Combine oats and almond milk in a large bowl.
Whisk egg yolk and butter together in a small bowl; stir into oat mixture.
Beat egg white in a glass, metal, or ceramic bowl until stiff peaks form.
Stir brown sugar and baking powder into the oat mixture. Fold beaten egg white gently into the batter.
Scoop 1/2 cup batter onto the preheated iron and spread to the edges. Close iron and cook until iron stops steaming, about 5 minutes. Repeat with remaining batter.