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Preheat the oven to 325°F. Line several cookie sheets with parchment paper, or lightly grease with non-stick vegetable oil spray.
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In a large bowl or the bowl of your stand mixer, beat together the butter and sugars until smooth.
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Beat in the egg, egg yolk, and vanilla.
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Whisk together the flour, oats, baking powder, baking soda, and salt, and add to the butter mixture in the bowl.
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Mix until everything is thoroughly incorporated. Scrape the bottom and sides of the bowl, and mix briefly.
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Stir in the chocolate chips.
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Decide what size cookies you want to make. A muffin scoop (1/4 cup) will make 20 large, palm-sized cookies. A tablespoon cookie scoop (4 teaspoons) will make 50 medium (2 3/4" to 3") cookies; and a teaspoon cookie scoop (2 teaspoons) will make 100 small (2 1/2") cookies.
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Scoop the dough onto the prepared baking sheets, leaving about 1 1/2" to 2" between cookies.
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Bake the cookies for 12 to 17 minutes, until they're a light golden brown, with slightly darker edges. Their middles may still look a tiny bit shiny; that's OK, they'll continue to bake as they cool on the pan.
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Remove the cookies from the oven, and as soon as they're set enough to handle, transfer them to racks to cool.
