Preheat oven to 350°F. 

Line a 9x13-inch baking pan with parchment paper, leaving overhang on two sides. Lightly grease exposed pan sides. Set aside. 

In a large bowl or stand mixer, cream together butter and sugars until light and fluffy. Mix in eggs, one at a time, followed by vanilla, salt and baking powder. Scrape bowl as needed. Add flour in small batches until fully incorporated. Stir in oatmeal and chocolate chips by hand. 

Transfer to prepared baking pan. Press down into an even layer, spreading into all corners. 

Bake for 25-30 minutes or until a toothpick inserted comces out clean. Cool completely in pan. 

When bars are fully cooled and set, carefully remove bars by pulling up and out on the overhanging parchment. Carefully cut into 12 or more equal pieces. Store in an airtight container for up to a week.