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Preheat oven to 350 degrees.
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Line a 10x15-inch jelly-roll pan with non-stick foil; arrange broken pretzel pieces to fit the bottom of the pan in a single layer. You'll want to cover as much of the bottom of the pan as possible, but gaps here and there are just fine.
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Melt the butter in a medium saucepan over medium heat and add the brown and white sugar.
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Stir to combine and bring mixture to a boil.
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Reduce heat and let the mixture gently bubble for 4 minutes and remove from heat.
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Carefully pour sugar/butter mixture evenly over the pretzel layer, spread to cover.
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Place pan in the oven and bake pretzels for 10 minutes.
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Once out of the oven, sprinkle white chocolate chips over the top.
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Wait about 2 minutes for chocolate to melt, then use a spatula to gently spread chocolate chips into an even layer on top of the pretzel layer.
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Sprinkle with M&M'S and festive holiday sprinkles.
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Cool to room temperature, then refrigerate for 2 hours.
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Peel foil from chilled bark and break into pieces.