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Preheat the oven to 375 degrees.
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Combine the cream cheese and butter in a large bowl and beat with an electric mixer until very smooth, scraping down the sides and bottom of the bowl as necessary.
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Add the egg, vanilla, and salt and beat to combine.
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Add the cake mix and beat until fully combined, again scraping down the bowl as needed.
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Use a rubber spatula to fold in the rainbow jimmies.
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Shape the dough into balls of about 2 tablespoons of dough. Arrange 1 inch apart on un-greased, unlined baking sheets and flatten lightly with your palm.
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Bake cookies 12 to 18 minutes, until they’re just beginning to turn light golden on top. Carefully remove to a wire rack to cool completely.
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Uneaten cookies can be stored in an airtight container at room temperature for 3 or 4 days or in the freezer (tightly wrapped) for 2 months.