Soft and chewy Congo Squares, or Chocolate Chip Cookies in a Pan. Recipe includes instructions on how to bake them in a 13x9 pan, or make them extra thick and chewy in a 9x9 pan.
Preheat oven to 350F (177C) and prepare either a 13x9 or 9x9 pan (13x9 for more servings, 9x9 for thicker, chewier congo squares) by lining with parchment paper (or lightly grease and flour).
Combine melted (cooled) butter and sugars in a large bowl, stir well.
1 cup butter, 1 cup light brown sugar, ¾ cup sugar
Add eggs, and egg yolk one at a time, stirring well after each addition.
2 large eggs + 1 egg yolk
Stir in vanilla extract.
1 ½ teaspoon vanilla extract
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
2 ¼ cups all purpose flour, 2 teaspoons cornstarch, ½ teaspoon baking powder, 1 teaspoon salt
Gradually stir dry ingredients into wet until completely combined. Stir in chocolate chips.
2 cups semisweet chocolate chips
Spread batter evenly into prepared pan. Bake on 350F (177C) until edges just begin to turn golden brown (30 minutes for 13x9 pan; 35 minutes for 9x9 pan).
Allow to cool before cutting and serving (don't cool too long, though -- these are amazing when served still slightly warm!)