In a large bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, granulated sugar, baking powder, baking soda, and salt. Beat mixture until combined, scraping sides of bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour. Stir in the oats and chocolate pieces.