Preheat the oven to 325°F.
Cover one extra large or two large cookie sheets with parchment paper.
In a medium bowl, mix together the flour, baking powder, and salt. Set aside.
Using an electric stand mixer, beat the melted butter, light brown sugar, white sugar, egg, egg yolk, and vanilla extract until smooth.
Add the flour mixture and stir just until the flour is combined.
Stir in the chocolate chips.
Divide the dough into 18 equal-sized balls (each should be about 60 grams or about 1½ tablespoons) and evenly space them out on the cookie sheet(s) about 2 inches apart.
Bake in the top portion of your oven for 18 to 20 minutes or just until the bottoms and sides are set and are lightly golden brown. The tops will be puffy and will not have much color. (Note: If you use two cookie sheets and two oven racks, be sure to rotate them halfway during baking.)
Cool the cookies completely on the cookie sheet(s) to maintain the chewy texture.
Store the cookies in an air-tight container.