Preheat oven to 350°. Line 2 large baking sheets with parchment paper; set aside.
In a large bowl with an electric mixer, beat butter and both sugars until fluffy. Mix in egg and vanilla until well blended. Combine flour, cinnamon, baking soda, salt, baking powder, nutmeg and cornstarch in a bowl. Stir into the butter mixture. Gently stir in oats until well blended.
Shape dough into rounded medium cookie scoop sized balls. About 1 ½ Tablespoon size. Place parchment lined baking sheet. Flatten slightly with the bottom of a measuring cup dipped in sugar. Bake for 8-10 minutes. Until edges are set and center is not quite wet. Let cool on baking sheet 3-5 minutes before removing to a wire rack to cool completely.
Notes
Old fashioned oats for this recipe works best.Be sure not to over bake.