- 6oz (3/4 Cup) Unsalted Butter, melted and slightly cooled
- 100 grams (1/2 Cup) Granulated/ Caster Sugar
- 150 grams (3/4 Cup) Light Brown Sugar
- 1 Large Egg + 1 Large Egg Yolk, at room temperature
- 2 Teaspoons Pure Vanilla Extract
- 166 grams (1 1/3 Cups) All-Purpose/ Plain Flour
- 100 grams (1 Cup) Old Fashioned Rolled Oats
- 1/4 Teaspoon Salt
- 1/2 Teaspoon Baking Powder
- 1 Teaspoon Cornstarch/ Cornflour
- 270 grams (1 1/2 Cups) Chocolate Chips or Chunks
- Preheat the oven to 350F/180C and line with parchment paper or foil and lightly grease a 9-inch square baking pan.
- In a medium-sized bowl whisk together the flour, baking powder, salt, and cornstarch. Set aside.
- In a large bowl whisk together the butter and sugars. Make sure to scrape down the sides of the bowl to ensure all is combined.
- Stir in vanilla, egg, and egg yolk, making sure to mix until fully combined. Scrape down the sides of the bowl again.
- Gently fold in dry ingredients and chocolate chunks until just combined. I like to save a few of the chocolate pieces to place on top of the bars before baking, just for aesthetics.
- Pour cookie dough into the prepared pan and gently press into an even layer, top with additional chocolate chips if desired. Bake the cookie bars for 23-26 minutes*. The edges should be golden brown and pulling from the edge of the pan, the center will still giggle a bit.
- Let the bars cool in the pan on a wire rack for 25 minutes then slice and serve. Store leftovers in an air-tight container for up to 5 days.
- Prep Time: 10
- Cook Time: 25
- Category: Dessert
- Method: Bake
- Cuisine: American
Keywords: oatmeal chocolate chip cookie bars, chocolate chip oat cookie bars