Preheat the oven to 350°F (180°C).
Butter or spray an 8-inch square baking pan.
You can line it with a piece of parchment paper, covering the pan's bottom and two long sides. The two short sides will remain unlined but greased. This will help you remove the bars from the pan.
In a large mixing bowl beat butter with both sugars for a minute.
Add the egg and incorporate well until you have a creamy mixture.
Add the vanilla and mix.
Sift the flour, baking powder, baking soda, and salt.
Add them to the butter mixture in 2 parts, mixing with a silicon spatula until combined.
Add the oats and incorporate well. It might take a minute or two.
Lastly, add the chocolate chips and mix to distribute well throughout the dough.
Pat the cookie dough in the prepared pan, even it out.
Bake for about 25 minutes, or until it’s slightly puffed, the top is dry and golden brown, but the center is still a little soft. Don’t let it firm up completely or the bars will be on the dry side.
Let cool on a wire rack.
Run a smooth-bladed knife around the edges to loosen up any bit that might be stuck, and lift the whole block with the help of the paper.
Store leftovers covered in plastic wrap or airtight container.