Preheat oven to 350°. Spray a 8 or 9" spring form pan with non stick cooking spray.
Melt butter in microwave for 30 seconds {there may be a little butter left unmelted but stir until it is completely melted}. Add granulated sugar to the melted butter and mix until creamy.
Add egg and vanilla until just incorporated. Be sure to not over mix or the cookie dough might get too stiff.
Then add the flour, baking soda and salt and mix until incorporated. It should form a ball of dough. Add the sprinkles and white chocolate chips to the dough until they are evenly distributed.
Put dough into springform pan and press into an even layer, to the outer edges of the pan. Bake for 20-25 minutes or until the cookie is just starting to turn golden brown. We do not want to over bake the cookie {it will continue to cook from the heat of the pan}. Remove it from the oven and let it cool in the pan completely.
Once cooled, remove the outer rim of the springform pan. Put frosting into a piping bag fitted with a large star tip. Applying even pressure, pipe stars around the edge of the cookie. Sprinkle frosting with additional sprinkles and white chocolate chips {if desired}.
Serve in wedges immediately or wait until the frosting is set up and cover with plastic wrap. Cookie will keep at room temperature for up to 4 days. Cookie will freeze for up to 3 months without frosting.