Prepare an 8-inch square baking pan with parchment paper. Allow the edges of the parchment paper to hang over the edge of the pan. This will help to remove the fudge from the pan when you're ready. Spritz with nonstick cooking spray.
Place the white chocolate chips into a microwave-safe bowl. Heat on high for 30 seconds, stirring vigorously so the chocolate continues to melt using the residual heat. Heat for an additional 15 seconds if needed.
Remove the foil lid from the frosting. Microwave for 30 seconds. Stir the melted frosting into the melted white chocolate.
Top with additional rainbow bits. Transfer the mixture to the prepared baking dish. Cover and chill in the refrigerator for 2 to 3 hours.
Use the parchment paper to lift the fudge from the baking dish. Cut into squares and serve or package for gifts.