Step 1

Preheat oven to 325 degrees.

Step 2

In a large mixing bowl, combine melted butter, brown sugar, and granulated sugar. Mix until smooth. Then add an egg, egg yolk, and vanilla extract and mix until combined.

Step 3

In a separate large bowl, combine flour, baking soda, cornstarch, and salt. Hand whisk until all of the ingredients are combined. Then gradually add the dry ingredients to the wet ingredients, mixing as you go. Once combined, mix in the chocolate chips.

Step 4

Line a 6u0022x10u0022 casserole dish with parchment paper, and add half of the dough to the bottom, using your hands to press it evenly across the bottom of the dish. Place chocolate bars on top of the bottom layer of dough, then cover the chocolate bars completely with the second half of the cookie dough. Press the cookie dough down around the edges to seal in the chocolate.

Step 5

Bake the bars on the middle rack until the tops are golden brown and a toothpick comes out clean-ish (obviously some chocolate will be on it), about 40 or 45 minutes. Don't over-bakeu002du002dthese are best when they're still gooey in the middle, and they set up a bit more after they've cooled.

Step 6

Allow the bars to cool slightly before cutting them, and enjoy!