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 flour, baking powder, nutmeg, and salt. Set aside.
Mix Yolks and Sugar: In a large mixing bowl, beat egg yolks with ¾ cup of sugar using an electric mixer until pale and creamy. Add the dry ingredients, then pour in the eggnog and vanilla extract. Mix until just combined.
Whip the Egg Whites: Clean the beaters thoroughly. In a separate bowl, beat egg whites on medium speed until soft peaks form. Gradually add the remaining ¼ cup of 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 the sweetened condensed milk, evaporated milk, and 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 the heavy cream, eggnog, and powdered sugar until stiff peaks form.
Assemble and Serve: Spread the whipped topping evenly over the chilled cake. Dust lightly with 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!