In a blender, process 1½ cups of oats into a fine "flour." Mix the oat flour with the cornstarch, cinnamon, baking powder, and salt.
In a separate bowl, mix butter and sugars with a hand mixer until light and fluffy. Beat in eggs and vanilla.
Add the oat mixture into the wet ingredients and mix until combined.
Gently fold in the remaining regular oats and the diced apple.
Place cookie dough balls on an ungreased cookie sheet and bake at 350° F for approximately 14 minutes, or until golden brown. Immediately remove the cookies and place them on a cooling rack. Makes about 3 dozen cookies. Enjoy!
Recipe by Food Fun Family at https://foodfunfamily.com/gluten-free-apple-spice-oatmeal-cookies/