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Preheat oven to 170c degrees.
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In a mixer bowl with a paddle attachment, place butter, sugar, dark brown sugar and salt and mix at high speed until a uniform creamy mixture is obtained.
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Scrub the edges of the bowl with a spatula and add in the egg and egg yolk.
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Continue to whip at high speed for 1-2 minutes until mixed.
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Add flour, baking soda, baking powder, cocoa powder, dark chocolate chips and white chocolate chips, and mix until large chunks of dough are formed.
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Using your hands, roll walnut-size balls out of the dough and place in increments on oven-lined baking pan.
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Flatten the cookies slightly and add some chocolate chips at the top of each cookie.
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Bake the cookies for 10-15 minutes or until they rise and set at the edges but are still slightly soft in the center.
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Cool completely and serve.