Preheat oven to 350 °F (177 °C) and liberally spray a 9x13-inch pan with cooking spray.
Combine flour, baking powder, and salt in a large bowl. Set aside.
In another bowl, beat the egg yolks with 3/4 cup of sugar on high speed with a hand mixer until they're pale yellow. Then mix in the milk and vanilla.
Gently pour the yolk mixture over the flour mixture and stir gently until all are combined.
In another bowl, beat the egg whites on high speed with a hand mixer until soft peaks form. Turn the speed down a little to add in the remaining 1/4 cup of sugar then turn it back up to high to beat until the egg whites are stiff but not dry.
Fold the egg white mixture into the batter very gently until just combined.
Pour into prepared baking pan and spread to even out the surface.
Bake for 35-45 minutes, or until a toothpick comes out clean.
Allow to cool completely in the pan.
In the meantime, combine eggnog, evaporated milk, and condensed milk in a measuring cup and mix together.
When cake is cool, pierce the surface with a fork several times (like I pierced it at least over 50 times).
Slowly drizzle milk mixture all over the cake.
Allow the cake to absorb the milk mixture for at least 3 hours in the fridge.
When ready to serve, using a hand mixer, beat the ingredients for the topping until whipped cream-like texture, about 5 minutes. Spread over the surface of the cake and sprinkle nutmeg on top.