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Preheat the oven to 375. Line a light-colored baking sheet with a silicone mat.
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In a medium bowl, beat the butter with an electric mixer on medium speed until fluffy, about 20 seconds.
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Add the sugars, and beat for about 30 seconds. The mixture will turn a pale color and be fluffy.
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Next, add the egg yolk and vanilla and beat until just combined.
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Whisk together the flour, salt, baking soda and baking powder in a separate bowl.
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Sprinkle the flour on top of the butter mixture, and beat just until combined.
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Stir in the white chocolate chips and Lucky Charms marshmallows.
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Scoop the dough into two giant balls, and space them evenly on the baking sheet.
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Bake for 10-12 minutes, removing the cookies from the oven when the edges just start to turn golden brown. Sprinkle the tops of the cookies with additional white chocolate chips and Lucky Charms.
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Let the cookies rest on the baking sheet for 1 minute before moving them to a wire rack to cool.