Brew the coffee and pour it into a shallow bowl. Set aside and let it cool completely.

In a saucepan, add 2 ¾ cups of the eggnog and heat it gently over medium heat until warm but not boiling. In a small bowl, whisk together the remaining 1/4 cup of eggnog with the cornstarch until completely smooth. Slowly whisk the slurry into the warm eggnog. Continue stirring constantly for 5-7 minutes, or until the mixture thickens. Do not let it boil. Once thickened, remove it from the heat and let it cool.

Once the coffee has cooled, quickly dip one side of each ladyfinger into the coffee for 1-2 seconds. Be careful not to soak them. Arrange 12 dipped ladyfingers in the bottom of a 9x9-inch dish.

Spread half of the cooled eggnog filling evenly over the layer of ladyfingers. Dip the remaining 12 ladyfingers and place them on top of the filling. Spread the remaining eggnog filling evenly over the second layer.

Spread the whipped topping evenly over the top layer of eggnog filling.

Cover and refrigerate for at least 4 hours to allow the layers to set and flavors to meld. Before serving, lightly sprinkle with cinnamon, slice, and serve chilled.