Step 1

Preheat your oven to 325°F. Line an 8-inch square baking pan with nonstick aluminum foil, or regular heavy-duty aluminum foil and spray it with nonstick cooking spray. Set the pan aside.

Step 2

In a small bowl, place the flour, xanthan gum, cornstarch, baking soda, and salt, and whisk to combine well. Set the bowl aside.

Step 3

In a medium-size, heavy-bottom saucepan, place the chopped butter, granulated sugar, and light brown sugar.

Step 4

Place the saucepan over medium-low heat and, stirring occasionally to prevent burning, melt the butter and sugar and bring to a gentle simmer.

Step 5

Remove the pan from the heat and set it aside to cool until no longer hot to the touch. You can immerse the bottom of the pan in some ice water, but take care not to let the water splash up into the pan and don’t cool so much that the butter resolidifies.

Step 6

In a large bowl, place the eggs, egg yolk, and vanilla, and beat with a handheld mixer until foamy and lighter in color (about a minute).

Step 7

Add the cooled butter and sugar mixture, and beat very well until the mixture falls off the beaters in ribbons (about 3 minutes).

Step 8

Add the flour mixture to the bowl, stir until the streaks of flour disappear, then beat briefly to ensure that everything is combined. The mixture will be thickly pourable and very sticky.

Step 9

Add all but about 1 ounce of the chocolate chips, and mix until the chips are evenly distributed throughout the cookie dough.

Step 10

Transfer the cookie dough to the prepared baking pan, and shake and spread it into an even layer in the pan.

Step 11

Scatter the remaining chocolate chips evenly over the top, and place the pan in the center of the preheated oven.

Step 12

Bake for about 40 minutes, or until the top no longer glistens or jiggles at all. Remove the pan from the oven.

Step 13

Allow the bars to cool in the pan at room temperature for at least 10 minutes before placing them, still in the pan, in the refrigerator to chill until firm to the touch.

Step 14

Remove the cooled bars from the pan by holding onto the foil. Slice into 9 equal pieces, and serve.