Preheat oven to 350°F and prepare a 9×13-inch baking dish with nonstick cooking spray or parchment paper
Mix white cake mix, 1 cup eggnog, ½ cup vegetable oil, 4 large eggs, 1 teaspoon vanilla extract, and ½ teaspoon nutmeg in a large bowl
Beat mixture on medium speed for 2 minutes until smooth
Pour batter into prepared baking dish and bake for 20-30 minutes until a toothpick inserted in center comes out clean
Let cake cool for 20-25 minutes
Use a skewer or fork to poke holes all over the cake about every inch
Allow cake to cool completely to room temperature
Whisk together 1 cup eggnog, ½ cup sweetened condensed milk, and ½ cup evaporated milk
Slowly pour tres leches mixture evenly over the cooled cake
Cover cake and refrigerate for at least 3-4 hours or overnight
Beat 2 cups cold heavy cream, ½ cup powdered sugar, and 1 teaspoon vanilla extract until stiff peaks form
Spread whipped cream over the top of the cake with an offset or rubber spatula
Sprinkle ground cinnamon and nutmeg over whipped cream before serving
Slice into squares and serve
