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Preheat the oven to 350° F. Line an 8-by-8-inch baking pan with aluminum foil and spray with cooking spray, or line with parchment paper. Set aside
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Cream together the butter, sugar, and brown sugar in the bowl of an electric mixer, until smooth and creamy. Add the egg and vanilla and mix until smooth. In a small bowl, mix together the flour, baking soda, and salt, and stir until just combined. Slowly add the dry ingredients into the wet ingredients, mixing on low to medium, until well combined.
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Fold in white chocolate chips and rainbow sprinkles, saving some to top the bars with at the end.
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Spread the batter into the prepared pan.
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Bake for about 20 to 25 minutes or the edges of the bar look golden brown or until a toothpick inserted in the center should come out clean. Sprinkle the top of the bars with the remaining white chocolate chips and rainbow sprinkles. Let the bars cool for about 30 minutes before cutting and serving. Enjoy!