Preheat oven to 325 degrees.
Line an 8-inch square baking dish (or a 9-inch pan for slightly thinner bars) with parchment paper or foil and lightly coat with cooking spray.
In a microwave-safe mixing bowl, combine the honey and butter. Microwave for 30 seconds, then stir well. Continue heating in 15–30 second intervals until the butter is fully melted and the mixture is smooth.
Add the oats, funfetti cake mix, vanilla extract, and salt to the melted honey mixture. Stir until everything is evenly combined and the oats are fully coated. Lastly, gently fold in additional rainbow sprinkles.
Transfer the mixture into the prepared baking dish. Using clean hands or the back of a spatula, press the mixture down firmly and evenly.
Bake the pan for 18–20 minutes, or until the edges turn light golden brown. Remove from the oven and allow the granola bars to cool completely. For faster cooling, place the pan in the refrigerator.
Once the bars are fully cooled and set, lift them out of the pan using the parchment paper. Slice into bars about 1 to 1½ inches wide.
Melt the white chocolate chips until smooth. For an easy drizzle, place the chocolate chips in a small zip-top bag and microwave for 30–45 seconds, or until melted. Cut a tiny corner off the bag and drizzle the melted white chocolate over the birthday cake granola bars. Allow the chocolate drizzle to set before serving.