Design Eat Repeat

0 Preheat oven to 350ºF. While preheating, be sure to read tips & tricks in post above for recipe success.
1 Warm unsalted butter ([adjustable]½ cup, i.e. 1 stick[/adjustable]) in a microwave-safe bowl or saucepan until it’s just melted.
2 In a large mixing bowl, combine the melted butter with the light brown sugar ([adjustable]½ cup) and granulated sugar (½ cup[/adjustable]). Stir until combined. (I recommend mixing this by hand rather than a mixer.)
3 Stir in the egg ([adjustable]1 large) and vanilla extract (2 teaspoons), followed by the baking powder (½ teaspoon) and salt (1 teaspoon[/adjustable]).
4 Mix in cornstarch ([adjustable]1 tablespoon) and all-purpose flour (1¼ cups), followed by the white chocolate chips (½ cup) & rainbow sprinkles (¼ cup[/adjustable]). Mix the dough until well-combined.
5 Generously grease a cookie cake pan. If you plan to invert the cake onto a tray (rather than serving in the pan), I highly recommend lining the bottom with parchment paper in order for the cookie cake to invert out of the pan easily.
6 Spread dough into an even layer in pan. Sprinkle on some extra sprinkles and white chocolate chips for added presentation. Bake for 16 to 18 minutes, or until edges are slightly golden brown. Do not overbake; it might look a little under-baked, but the residual heat from the pan will continue to bake the cookie even after you take it out of the oven.
7 Cool completely and top with buttercream frosting, if desired (this cookie cake is so yummy even without the frosting!).
9 To decorate, use a piping bag and large star tip to create a simple vanilla buttercream border. Immediately after piping, top with sprinkles
To Make Frosting:
11 Beat the softened unsalted butter ([adjustable]½ cup, i.e. 1 stick), vanilla extract (2 teaspoons[/adjustable]), and pinch of salt on medium-high speed for 1 minute until smooth. Turn the mixer to low speed then slowly add in the powdered sugar ([adjustable]2½ cups[/adjustable]). You may need to stop and scrape down sides. One tablespoon at a time, add milk ([adjustable]2-3 tablespoons,[/adjustable] as needed). Beat on high for 2-3 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tiny bit of milk until you get to the desired consistency. This will make just enough frosting to pipe a generous border along the edges, as well as write a message in the middle.