Preheat oven temperature to 375°. Prepare baking sheets with silicone mat or parchment paper.
Melt butter and set aside to slightly cool.
In a medium bowl, whisk flour, corn starch, baking powder, and salt. Set aside.
Using an electric mixer, in a large bowl, combine melted butter, brown sugar, and white sugar. Cream until well combined on high (about 1 minute). The mixture will be dark brown and quite thin.
Add eggs and yolk. Beat on medium for about 30 seconds until mixture starts to smooth out and lighten in color.
Slowly add about ½ cup of the dry mixture at a time and beat on low after each addition. As soon as all of the flour has been added, stop beating.
Stir in the chocolate chips.
Measure about ¼ cup (or 60g) cookie dough balls and place them on the prepared baking sheet. Add a few extra chocolate chips on top.
Bake for 10-12 minutes. Remove from oven when the edges are slightly golden brown and the top is still light in color. Let cool on the tray for 5 minutes and then move to a wire rack to completely cool.
Notes
Store in an airtight container for up to 3 days or freeze for up to 3 months.