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Melt butter in medium saucepan over low heat.
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Remove from heat and add sugar, whisking until well combined.
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Add eggs and vanilla, whisking until well combined.
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Whisk in flour.
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Remove half of mixture into a separate bowl.
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To the saucepan, add cocoa and peanut butter chips, stirring well.
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To the bowl, add peanut butter and chocolate chips.
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Spread chocolate mixture into bottom of greased 9x13" baking pan.
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Spoon peanut butter mixture over top and swirl with a knife.
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Bake at 350 degrees for 25-30 minutes. Toothpick inserted should come out with fudgy crumbs. Do not over bake!
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Cool wire rack.