Preheat the oven to 350ºF. Spray a 9" springform pan with non-stick spray*. Set aside.
In a medium size bowl, whisk together the flour, baking soda, salt, and cinnamon. Set aside.
In a large bowl, whisk together the butter, sugar, peanut butter, egg, and vanilla until smooth.
Pour the dry ingredients into the wet ingredients and stir until cookie dough comes together. Press the cookie dough evenly into the prepared pan. Bake for 21-23 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.
Remove cooled cookie cake from springform pan and place on serving platter. Fill a piping bag with the frosting of your choice and decorate the cookie cake. I like to use a Wilton 1M tip. 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.
*Alternatively, you may use a 9" pie plate or a 8" or 9" round or square baking pan.