1) Preheat the oven to 325°F. Line several cookie sheets with parchment paper, or lightly grease with non-stick vegetable oil spray.
2) Beat together the butter and sugars until smooth.
3) Add the egg, egg yolk, and vanilla one at a time, beating well after each.
4) Whisk together the flour, oats, baking powder, baking soda, and salt, and add to the butter mixture in the bowl.
5) Mix until everything is thoroughly incorporated. Scrape the bottom and sides of the bowl, and mix briefly.
6) Stir in the chocolate chips.
7) 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.
8) Scoop the dough onto the prepared baking sheets, leaving about 1 1/2" to 2" between cookies.
9) Bake the cookies for 12 to 15 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.
10) Remove the cookies from the oven, and as soon as they're set enough to handle, transfer them to racks to cool.
Yield: 20 to 100 cookies, depending on size.