Preheat your oven to 350°F (175°C). Prepare a 9x13 inch baking pan by greasing it lightly or lining it with parchment paper for easy removal after baking.
In a medium-sized mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, ground nutmeg, ground cinnamon, and salt until well combined. Set this dry mixture aside for later use.
In a large mixing bowl, using an electric mixer on medium speed, cream the softened butter, packed brown sugar, and granulated sugar until the mixture is light in color and fluffy in texture, about 3-5 minutes.
Add the eggs to the butter-sugar mixture one at a time, beating well after each addition until fully incorporated. Then mix in the pure vanilla extract until combined.
Gradually add the dry ingredients to the wet mixture in three parts, alternating with the eggnog. Start and finish with the flour mixture, mixing gently until just combined after each addition to avoid overmixing.
If desired, fold in the chopped pecans or walnuts gently for an added crunch and flavor.
Pour the cookie bar batter into the prepared baking pan and spread it out evenly with a spatula or the back of a spoon.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the cookie bars cool in the pan on a wire rack for at least 10 minutes. Once cooled enough to handle, cut them into generous squares.
For an optional festive touch, top each square with a dollop of whipped cream just before serving.
Notes
For an attractive finish, lightly dust with powdered sugar and garnish with nutmeg or a cinnamon stick.