Preheat the oven to 180°C or 350°F. Spray a 9-inch circle cake pan or a spring form pan with oil, and line the bottom and sides with parchment paper.
In a stand mixer, with a paddle attachment, cream the butter and the sugars until fluffy. Add the egg, cinnamon, eggnog, and mix.
Fold in the flour, baking soda, baking powder and salt. Once everything is smooth and combined, fold in the chocolate chips.
Evenly spread into the prepared pan, and if desired, sprinkle extra chocolate chips on top.
Bake for 25-30 minutes, or until the top is golden brown.
Allow the cookie cake to cool in the pan before removing. Let the cookie cake fully cool, transfer to a serving plate and decorate with the eggnog buttercream.