In a large mixing bowl, beat butter until it's creamy. Add the sugars and beat on medium speed until it's well combined, about 2 minutes.
½ cup unsalted butter at room temperature (113g), ½ cup light brown sugar (100g), ¼ cup granulated sugar (50g)
Add the egg and vanilla and beat until well combined, scraping down the sides of the bowl as needed.
1 large egg, 1 teaspoon vanilla extract
Add the flour mixture and the chocolate chips and beat just until combined. The dough will be thick.
1¼ cups semi-sweet chocolate chips (212g)
Drop the dough into the prepared pan. Wet your hands and press the cookie dough into an even layer. If you want, add a few more chocolate chips on top.
Bake for 15 to 20 minutes or until the cookie is lightly browned. Don't overbake or it will get hard.
Allow the cake to cool completely in the pan on a wire rack before frosting.
Make the vanilla buttercream frosting and decorate
Beat the butter on high speed until it's for about 3-4 minutes. Scape the sides of the bowl a few times during this time.
¼ cup unsalted butter at room temperature (57g)
Add the powdered sugar in 2-3 batches, while the mixer is still running. Start on low speed and gradually increase speed to medium. Mix well after each addition.
1¼ cups powdered sugar sifted (150g)
Add the vanilla and the milk, one tablespoon at a time (you may not need all of the milk) and beat until you reach your desired consistency. If it's too thick, add a tablespoon more milk, if it's too thin, add 1-2 tablespoons more powdered sugar.
½ teaspoon vanilla extract, 1-2 tablespoons milk
Beat for another 3-4 minutes on high speed so that it gets lighter in color.
Stir the frosting with a rubber spatula for about a minute to remove any air bubbles and make it more spreadable.
Transfer the frosting to a piping bag with a tip.
Run a knife around the edge of the cookie, release the ring, and transfer it to a serving plate.
Spread some frosting all over the surface and pipe some swirls around the edge.
Decorate with your favorite Christmas sprinkles.
Enjoy!
Notes
Tips for Making the Perfect Cookie Cake
I use a springform pan because it's easier to remove the cookie, but you could use a regular 9 or 10-inch baking pan.
Weigh your ingredients for the best results. Room-Temperature Ingredients: Use room-temperature ingredients to ensure even mixing.
Avoid Overmixing as it can make the cookie hard instead of soft.
Slightly underbaking keeps the cookie soft; overbaking makes it hard and dry.
Allow the cookie to cool completely to prevent the frosting from melting.