Preheat oven to 350°F. Arrange a large baking sheet with parchment or silicone baking mat.
1
Beat butter, brown sugar, and granulated sugar with a hand-held mixer in a medium bowl until light and fluffy. Add egg yolk and vanilla and mix well, scraping bowl sides as needed.
2
Mix flour, baking soda, and salt until combined. Then fully incorporate the rolled oats and semi-sweet chocolate chips into the dough.
3
Two tablespoon-sized pieces of dough should yield 8-9 cookies on the prepared baking sheet.
4
Baking at 350°F for 9-11 minutes. Remove cookies from oven and cool on baking sheet for 5 minutes.