Preheat oven to 350 degrees. Grease a 9 inch square baking pan and line the bottom with parchment paper.
In a large bowl, whisk flour, baking soda, cornstarch, and salt. Set aside.
In a medium bowl, whisk melted butter, brown sugar, and granulated sugar until fully combined. Add the egg, egg yolk, and vanilla. Pour the wet ingredients into the dry ingredients and stir until combined. Fold in 1 cup of the chocolate chips.
Pour the dough into the prepared baking dish and sprinkle the remaining 1/4 cup chocolate chips over the top, lightly patting them into the top of the dough.
Bake for about 30 minutes until lightly browned. Let cool. Cut into squares and serve.
Notes
Katie's Tips:*Sometimes I line the whole pan with foil with the sides overhanging the baking pan. After the bars cool, I pull the bars out of the baking pan and cut. *I store the bars in a covered container in the fridge for up to a week. *For a more upscale presentation, sprinkle the top with chocolate chunks instead of chocolate chips.*This recipe was originally found and adapted from Sally's Baking Addiction's recipe found here.