Homemade Cookie Cake with Cookie Cake Frosting

Preheat the oven to 350ºF (170ºC). Spray a 9" springform pan³ with non-stick spray. Set aside.
In a large bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.
2 and ¼ cup (270g) all-purpose flour, 2 teaspoons (5g) cornstarch, 1 teaspoon baking soda, ¾ teaspoon salt
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.
¾ cup (171g) unsalted butter, 1 cup (200g) firmly packed brown sugar, ¼ cup (50g) granulated sugar, 1 large egg + 1 large egg yolk, 2 teaspoons vanilla extract
Pour the wet ingredients into the dry ingredients and stir until no flour lumps remain. Fold in the chocolate chips. Press the cookie dough evenly into the prepared pan. Bake for 23-25 minutes until edges begin to brown. You may cover the cake with foil to prevent browning, if preferred. Remove from oven and allow to cool on a wire rack completely before decorating with frosting.
1 cup (170g) chocolate chips1

In a small saucepan over medium heat, melt the unsweetened baking chocolate and the butter, stirring constantly to prevent scorching or burning. Remove from heat and allow to cool for 15 minutes.
1 ounce (14g) unsweetened baking chocolate2, 2 Tablespoons (28g) unsalted butter
In a medium size bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine the cooled chocolate/butter mixture, vanilla extract, and milk or cream (starting with 2 Tablespoons). Turn the mixer down to 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.
½ teaspoon vanilla extract, 2 to 3 Tablespoons (30-45mL) milk or cream, 1 cup (120g) powdered sugar, pinch of salt
Remove cooled cookie cake from springform pan and place on serving platter. Fill a piping bag with the frosting and decorate the cookie cake. I like to use a Wilton 1M tip. If using, add sprinkles before frosting sets. Store leftovers covered tightly at room temperature for up to 5 days. Undecorated cookie cake may be frozen, up to 2 months. Thaw in refrigerator overnight and allow to come to room temperature before decorating.