Preheat oven to 350 degrees. Grease a 9x13 inch pan. For a thicker cake, you can use a 9x12 inch pan or glass dish. For a half batch, use an 8-inch square dish.
Sift or whisk flour with baking powder and set aside. If using salt, whisk that it too.
In a large bowl with clean beaters, beat egg whites until foamy. Gradually add sugar and continue beating until thick and white. Beat in yolks, one at a time.
If using vanilla, add it to the milk.
Add flour mixture and milk to te egg mixture, mixing by hand OR beating on low just until blended. Pour batter into prepared pan and bake until edges are golden brown, about 30 minutes. Remove from oven and let cool on a rack.
Prepare the milk mixture by whisking together the sour cream and condensed milk, then pouring in the evaporated milk and whole milk. Try to use room temperature ingredients to avoid little lumps of sour cream, but don't worry if you have a few because they will be absorbed into the cake.
Using a skewer, poke holes all over the warm cake. Gradually pour milk mixture over cake and let sit until all mixture is absorbed. Let sit at room temperature for another 30 minutes or so to cool, then cover with plastic wrap and chill for several hours or overnight. Prepare whipped topping shortly before serving.
Beat cream with electric mixer until it begins to thicken. Gradually add sugar and vanilla and beat until stiff peaks form. Cover cake with whipped cream and continue to chill briefly before serving. You can also garnish it with caramel sauce, cinnamon, whatever you like! And if you have some berries on hand, be sure to put some on each plate as an accompaniment.
Cut cake and serve.
Notes
The salt is optional. I find it ramps up the flavor a bit, so I add it. If you leave it out, you'll still have some saltiness from the baking powder and the sour cream.