Preheat your oven to 350°F. Prepare an 8×8-inch square pan by lining it with parchment paper.

Whisk together the all-purpose flour, baking soda, cornstarch, and kosher salt in a large mixing bowl.

Combine the melted unsalted butter with creamy no-stir peanut butter, light brown sugar, and granulated sugar in a medium bowl. Whisk until smooth and creamy, then add in the pure vanilla extract, large egg, egg yolk, and milk.

Mix the wet ingredients into the dry ingredients using a rubber spatula until just combined. Gently fold in the peanut butter chips.

Spread the dough into the prepared pan, pressing it into the corners and smoothing the top.

Bake in your preheated oven for 32-35 minutes, checking for doneness.

Cool the bars in the pan on a wire rack for at least an hour. Once cooled, cut into 16 squares.