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Melt butter in saucepan over medium heat. Cook, stirring often, about 6 minutes, until it becomes brown in color. It will foam, reduce and then foam again. Once it’s done cooking the foam is light brown and it will dissipate after you remove from the heat. Once it starts to brown, keep stirring so it doesn’t burn. Let stand for 5 minutes to cool.
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Pour butter into bowl of electric mixer. Add both sugars and mix on low until combined.
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Combine flour, baking soda and salt in a medium bowl. Stir and set aside.
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Turn the mixer on low. Add eggs, one at a time, mixing completely. Mix in vanilla extract. Add flour mixture to the wet ingredients and mix until combined. Stir in chocolate chips.
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Line a 9x13 baking pan with foil and spray with cooking spray for easy removal. Press half of the cookie dough into prepared pan. Pour the sweetened condensed milk over the top, leaving ¼ inch edge around the pan. Then break up the chocolate bar and drop pieces over the top. Break up the remaining cookie dough and place it on top of the milk. Press with your hands to flatten. It will mostly cover but not completely cover the bottom layer.
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Refrigerate pan for 30 minutes.
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Preheat oven to 350 F. Bake for about 30-35 minutes until the tops of the bars start to get golden brown. Cool completely before slicing into bars.