Preheat your oven to 350°F and line an 8x8 inch baking pan with parchment paper, leaving some overhang on the sides to easily lift out the bars later.
In a medium-sized bowl, combine the graham cracker crumbs, sugar, and melted butter for the crust. Mix until the crumbs are evenly coated with the butter.
1 1/2 cups graham cracker crumbs, 1/4 cup granulated sugar, 1/2 cup unsalted butter
Press the graham cracker mixture firmly into the bottom of the prepared pan to create an even crust. You can use the back of a measuring cup or your hands to press it down. Bake in the oven for 10 minutes.
Cheesecake layer
Reduce the heat to 325F and move on to making the cheesecake filling.
In a large bowl, using an electric mixer or a stand mixer, beat the softened cream cheese and sour cream until it's smooth and creamy.
16 oz cream cheese, 1/4 cup sour cream
Add the granulated sugar and continue to beat until well combined.
1/2 cup granulated sugar
Mix in the eggs one at a time, ensuring each egg is fully incorporated before adding the next.
2 large eggs
Add the eggnog, vanilla extract, ground nutmeg, and flour to the cream cheese mixture. Mix until the batter is smooth and all ingredients are fully combined.
Pour the cheesecake mixture over the graham cracker crust in the pan. Smooth the top with a spatula.
Bake in a water bath the preheated oven for about 40-45 minutes, or until the edges are set and the center is slightly jiggly.
Remove the cheesecake bars from the oven and let them cool in the pan on a wire rack. Once they've cooled to room temperature, transfer them to the refrigerator and chill for at least 3-4 hours, or overnight.
Once the bars have chilled and set, use the parchment paper overhang to lift them out of the pan. Cut into squares.
If desired, top each bar with a dollop of whipped cream and a sprinkle of ground nutmeg before serving.