Preheat the oven to 350ºF. Spray a 9" springform pan with non-stick spray*. Set aside.

In a large bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.

In a medium size bowl, whisk together the butter, brown sugar, and granulated sugar until smooth.

Whisk in the egg, egg yolk, and vanilla extract until completely combined.

Pour the wet ingredients into the dry ingredients and stir until no flour clumps remain.

Bake for 23-25 minutes until edges begin to brown.

Remove from oven and allow to cool on a wire rack completely before decorating with frosting.

1 tub of Chocolate Fudge Frosting or make it home-made.

In a small saucepan over medium heat, melt the unsweetened baking chocolate and the butter. Allow to cool for 15 minutes.

Using a mixer, combine cooled chocolate/butter mixture, vanilla extract, and milk or cream in a bowl.

Turn the mixer onto low and slowly add the powdered sugar.

Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add salt to taste.

Remove cooled cookie cake from springform pan and place on serving platter.

Fill a piping bag with the frosting and decorate the cookie cake.

If using, add sprinkles before frosting sets.

Cookie cake remains fresh covered tightly at room temperature for up to 4 days.

Undecorated cookie cake may be frozen, up to 2 months.

Thaw in refrigerator overnight and allow to come to room temperature before decorating.