12tablespoonsunsalted butter, 1 1/2 sticks, melted and cooled slightly
1cuplight brown sugar, 7 ounces
1/2cupgranulated sugar, 3 1/2 ounces
1large egg
1large egg yolk
2teaspoonsvanilla extract
1cupchopped Kit Kat Bars
1cupmilk chocolate chips
Instructions
Heat oven to 325 degrees F. Butter the bottom and sides of a 9×13 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. Place another piece of parchment paper in the same manner, perpendicular to the first sheet. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In a medium bowl, whisk together flour, salt, and baking soda; set aside.
In a large bowl, whisk together melted butter and sugars until combined. Add egg, egg yolk, and vanilla and mix well. Using a rubber spatula, fold dry ingredients into egg mixture until just combined. Fold in chopped Kit Kat Bars and milk chocolate chips. Transfer to prepared baking pan and smooth the top with a spatula.
Bake until top is light golden brown, slightly firm to the touch, and edges start pulling away from sides of pan, 27 to 30 minutes. Cool on wire rack to room temperature. Remove bars from pan by lifting parchment overhang and transfer to cutting board. Cut into 2-inch squares and serve.