Preheat your oven to 350-F.
Lightly spray the bottom and sides of your 9 x 13 baking dish.
In a medium-sized bowl, whisk the all purpose flour, baking powder, and salt together. Set this aside.
In the bowl of a stand mixer with whisk attachment (or using a hand mixer) beat the egg whites on high until they're forming soft peaks.
Beat in the granulated white sugar until it is all combined and is glossy.
Add the egg yolks, beating them in one at a time until well mixed.
Add half the flour mixture to the eggs, then add 1/2 cup of milk until well mixed. Add the remaining flour and scrape down the sides of the bowl and remix to incoprorate all the flour.
Pour the batter into your prepared 9 x 13 cake pan and bake at 350-F for about 25 minutes or until the top is golden brown and an inserted toothpick comes out clean. Let the cake cool completely before moving on.
Using a fork or a skewer, poke holes all over the top of the cake going down nearly to the bottom of the cake.
Combine the eggnog with the remaining milk and mix well. Pour this mixture over the top of the cake, letting it seep in. Run a thin knife blade around the outside of the cake so the eggnog can seep into the edges, too. Cover and refrigerate for about 4 hours or up to 3-4 days.