In a mixing bowl, combine butter and sugar until well mixed.

Add egg, yolk, and vanilla and stir until combined.

Add flour, cornstarch, and salt and stir until combined.

Stir in the chocolate chips.

Line a cookie sheet with parchment paper or a silicone baking mat. Form approximately 1 tablespoon balls of dough and place on lined cookie sheet. Refrigerate dough balls for at least 30 minutes.

Preheat oven to 325 F. Line two cookie sheets with non-stick foil or a silicone baking mat. Place cooled dough balls approximately 1.5"-2" apart on lined cookie sheets. Using the palm of your hand, press down to flatten the balls to about ½" thick.

Bake at 325 for 8-10 minutes. Cool on the baking sheet for approximately 5 minutes, then transfer cookies to wire rack to cool completely.