Preheat oven to 350°F Grease a 9-by-13-inch glass baking pan with butter. In an electric mixer fitted with the whisk attachment, combine egg whites, baking soda, and salt. Beat until soft peaks form. Do not overbeat.
Add egg yolks to the egg white mixture. Beat just until completely combined. With the mixer running on low speed, slowly add the sugar until combined. Remove the bowl from the mixer. Fold in the melted butter by hand using a rubber spatula or hand whisk.
Sift 1/4 cup flour on top of the cake mixture. Fold in to combine. Sift and fold in remaining flour 1/4 cup at a time. Don’t overfold or you will lose volume. Pour the batter into the prepared pan. Bake until cake is golden and a cake tester inserted into the middle comes out clean, about 20 to 25 minutes.
While the cake is baking mix together the milk, evaporated milk, condensed milk and vanilla.
When the cake is done remove it from oven and place the pan on a wire rack. Quickly poke some small holes all over the cake using a cake tester or a toothpick.
Pour the milk mixture over the hot cake. It should take about 5 minutes for all the milk to absorb. Let the cake stand in the pan until cool. Cover the cake with plastic wrap and place in the refrigerator for at least 4 hours but preferably overnight.
When ready to serve frost the cake with either Italian Meringue frosting or whipped cream. Serve with fresh fruit if desired.