Prep work: Preheat oven to 325 degrees Fahrenheit. Cut two strips of parchment paper to fit across the interior of an 8 to 9-inch square baker. Criss-cross the papers at the bottom of the baker and fold the ends up the sides of the baker (see photos). If you need to make your own oat flour, blend 1 and ¾ cups oats in a blender or food processor until ground into a fine flour.