• Preheat the oven to 350 degrees.
• Prepare a 9-inch cake pan. Spray the pan with non-stick cooking spray. Cut a piece of parchment paper to fit the bottom of the pan. Add the paper and then lightly spray that as well.
• In a medium bowl, whisk together the flour, baking soda, and corn starch. Set aside.
• In the bowl of a mixer, add the butter, brown sugar, and granulated sugar.
• Beat the butter and sugars at low speed until creamy, about 2 minutes.
• Add the eggs one at a time, beating between additions.
• Mix in the vanilla.
• Slowly add the dry ingredients to the ingredients in the mixing bowl. You can add it in two or three additions mixing briefly to incorporate. Make sure you scrap down the sides and bottom of the bowl.
• Add in the chocolate chips and mix by hand. I was running out of chocolate chips, so I used a bit more than 1 cup normal size chips in the batter and then sprinkled about 1/4 cup mini chocolate chips onto the surface once the dough was in the pan.
• Scoop the batter into the prepared pan, smoothing the surface with a spatula. If you choose to add some chips to the top, sprinkle them over the surface and press them in lightly.
• Bake for about 30 minutes until golden. The cake will pull away from the sides and a toothpick inserted into the middle will come out clean. Start checking for doneness at about 25 minutes as all ovens are different.
• Let the cake cool in the pan on a rack for a few minutes, then flip it out onto a wire rack to cool completely.

Buttercream Frosting

• You will need piping bags and tips for this part. The cookie cake is fine without frosting, so you can skip this if you don't have the equipment.
• Cut the softened butter into chunks and add into a mixing bowl.
• Add the confectioners sugar and beat at medium speed. I started with two cups and then tested for texture and taste. I ended up using 2 1/2 cups. Always make additions slowly so you don't overdo.
• Add the vanilla and heavy cream and mix until incorporated.
• If you find the frosting to be too thin or soft, you can add a little more of the powdered sugar. Remember to add the sugar in small amounts.
• Once the frosting is ready, you can color it. I separated mine into three equal portions, placing each in a bowl.
• Add gel food coloring into each bowl. I only needed one drop of each color. Make sure you use a separate spoon to mix each one.
• The cake must be completely cool before you frost it.
• Get the piping bags and tips ready. Fill each bag with a separate color frosting and decorate.
• Store your cookie cake in an airtight container.