cinnamon sugar coating: 1 and ½ Tablespoons (13g) granulated sugar + 1 and ½ teaspoons ground cinnamon
1Tablespoon(14g) unsalted buttersoftened to room temperature
½Tablespoon(8g) milk or cream
1 and ½cups(180g) powdered sugar
In a small bowl, whisk together the melted butter and egg white until combined.
Add both sugars and the cinnamon and whisk again until smooth. Set aside.
Preheat the oven to 350ºF (177ºC). Spray a 9" springform pan with non-stick spray*. Set aside.
In a large bowl, toss together the flour, cornstarch, baking soda, cinnamon, and salt. Set aside.
In a medium size bowl, whisk together the butter and 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 with a large spatula until no flour clumps remain. Pour the cinnamon ribbon batter into the bowl and stir a few times to create swirls (about 5 to 7 strokes). Batter will be very wet. This is ok.
Pour the cookie dough into the prepared pan and press evenly to the edges of the pan with the spatula. Sprinkle with cinnamon sugar coating, then bake for 30 minutes until edges are set and a toothpick inserted in the center comes out mostly clean. Remove from oven and allow to cool on a wire rack completely before decorating with frosting.
In a medium bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine the butter, milk or cream, and vanilla extract. Turn the mixer onto low and slowly add the cinnamon, salt, and 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. I like to use a Wilton 1M tip. Sprinkle with additional cinnamon sugar mixture, if desired. 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.
*Springform pan: alternatively, you may use a 9" pie plate or a 8" or 9" round or square baking pan.