Preheat the oven to 350F. Line a large baking tray with parchment paper or a silpat liner.
Melt the butter a little bit in the microwave.
Add the butter, brown sugar, granulated sugar, egg yolk, and vanilla to a medium bowl, and use a fork or whisk to stir until smooth.
Add the flour, cornstarch, baking soda, and salt, and stir until well-combined.
Stir in 5 tablespoons chocolate chips.
Divide the dough into 2 equal balls and place them on the prepared baking tray.
Sprinkle the remaining 1 tablespoon chocolate chips on top of the cookie dough balls.
Bake until the cookies are light golden, about 12 to 14 minutes. They should still look a touch doughy in the center when you take them out of the oven.
Let the cookies cool on the baking tray.