Step 1

Preheat oven to 325 degrees. Butter an 8x8 inch baking pan and set aside.

Step 2

To warm the eggnog: place eggnog in microwave container and warm in 15 second increments until lukewarm (about 1 to 1 ¼ minutes)

Step 3

Combine the room temperature egg whites and distilled white vinegar and whip eggs with a mixer on the highest speed until forming stiff peaks. Set aside (note: the vinegar helps stabilize the peaks which is a key to success especially at high altitude).

Step 4

In a second bowl, mix together the egg yolks and sugar until fully combined and sunny yellow! Add melted butter; mix on low speed until the mixture is fully combined.

Step 5

Add the flour very gradually to the batter while mixing, to allow the flour to integrate with the rest of the batter.

Step 6

Add the rum and vanilla to the warmed eggnog. While keeping the mixer running on low speed, trickle the eggnog mixture into the batter allowing it to incorporate with the batter. By trickling the eggnog, it has time to mix in easily. Note: by the end the batter will be very watery and should be so don't worry!

Step 7

By hand, fold in egg whites one fourth at a time, stirring to combine with the liquid batter. There will be small lumps of egg white in the end and the batter will still be fairly liquid.

Step 8

Pour into the prepared pan and place in the preheated oven for 50-55 minutes. NOTE: The cake will have a browned top but may seem to have some movement when removing it from the oven which is normal.

Step 9

Allow to cool on a cooling rack for until completely cool (about 3 hours) OR allow to cool on the rack for 30 minutes, then cover with foil (shiny side out) and place in the refrigerator for at least an hour. Cut, sprinkle with confectioner's sugar and freshly grated nutmeg; serve!