Whisk flour and baking soda together in bowl and set aside.
Melt butter in a saucepan, or in the microwave.
Pour the melted butter in a bowl and whisk in brown sugar, granulated sugar, vanilla and salt.
Whisk in egg and yolk for 30 seconds until smooth and no lumps remain.
Let mixture rest for 3 minutes
Whisk for 30 seconds and let rest 3 minutes. Whisk again until smooth.
Using spoon, or rubber spatula, fold in flour mixture just until combined.
Grease baking pan and then transfer cookie dough to the pan. Spread the dough evenly and then top with chocolate chips.
Bake for 2 minutes and remove from oven. Using a toothpick, marbleize the top of the cookies by swirling the toothpick through the softened chocolate chips. If you find that the chips have not melted enough, put back in oven for another 30 seconds.
Return baking pan to oven and bake for an additional 15-18 minutes. Remove from oven when edges are just starting to turn color and center is starting to center.
Notes
Adjust the amount of chocolate chips you use according to your preference. However, the more you use means less marble effect on top.
If you prefer a more cake-like bar, increase the baking soda to 1 teaspoon and use 2 full eggs instead of 1 egg and 1 yolk.
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