Preheat the Oven: Preheat the oven to 350°F. Do not grease the baking dish. Line only the bottom with parchment paper if desired.
Combine Dry Ingredients: In a medium bowl, whisk together the 1 cup all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon ground nutmeg, and 1/4 teaspoon salt. Set aside.
Mix Yolks and Sugar: In a large mixing bowl, beat 5 large egg yolks with 3/4 cup granulated sugar using an electric mixer until pale and creamy. Add the dry ingredients, then pour in 1/2 cup eggnog and 1 teaspoon pure vanilla extract. Mix until just combined.
Whip the Egg Whites: Clean the beaters thoroughly. In a separate bowl, beat 5 large egg whites on medium speed until soft peaks form. Gradually add the remaining 1/4 cup granulated sugar, then increase to high speed and beat until stiff peaks form.
Fold and Bake: Gently fold the whipped egg whites into the yolk batter in three additions, being careful not to deflate the mixture. Pour the batter into an ungreased 9x13-inch baking dish. Bake for 25–30 minutes, or until the top is golden and a toothpick inserted in the center comes out clean.
Cool Completely: Let the cake cool in the pan for about 1 hour, or until it reaches room temperature.
Soak the Cake: In a medium bowl, whisk together 1 (14-ounce) can sweetened condensed milk, 1 (12-ounce) can evaporated milk, and 1/4 cup whole milk. Using a fork, poke holes all over the cooled cake. Slowly pour the milk mixture evenly over the cake. Cover with plastic wrap and refrigerate for at least 3 hours, or overnight, to allow the cake to absorb the milk.
Make the Whipped Topping: In a large chilled mixing bowl, beat 1 1/2 cups heavy whipping cream, 1/4 cup eggnog, and 3 tablespoons powdered sugar until stiff peaks form.
Assemble and Serve: Spread the whipped topping evenly over the chilled cake. Dust lightly with Ground cinnamon and slice into squares. Serve cold and enjoy every creamy, festive bite!
Notes
Make-Ahead Tips: Bake and soak the cake a day ahead to let the flavors develop overnight. Add the whipped topping right before serving.Storage: Keep covered in the refrigerator for up to 3 days.Freezing: For best results, freeze the cake before adding the milk mixture or topping. Wrap tightly in plastic wrap and foil, and freeze for up to 2 months. Thaw overnight in the fridge before completing the recipe.Reheating: This cake is meant to be served chilled — no reheating needed!