1. Beat egg yolks, butter, and sugar at medium speed with an electric mixer 2 minutes or until mixture is creamy.
2. Combine flour, baking powder, and salt. Add to egg yolk mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in vanilla.
3. Beat egg whites until stiff; fold gently into batter. Pour batter into a greased and floured 13- x 9-inch pan.
4. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Let stand 10 minutes.
5. Pierce top of cake several times with a small wooden skewer. Stir together condensed milk, evaporated milk, and cream; gradually pour and spread over warm cake. (Pour about 1/4 cup at a time, allowing mixture to soak into cake before pouring more.) Let stand 2 hours; cover and chill overnight, if desired. Spread top of cake with Fluffy Frosting before serving. Garnish, if desired.
Tricks to Perfect Tres Leches: Use tips from Test Kitchens Director Lyda Jones Burnette to make this cake turn out perfectly.
* Let the cake cool in the pan 10 minutes after removing it from the oven.
* While it's still warm, poke holes in the cake with a wooden pick or skewer.
* Slowly pour the milk mixture over the cake--about 1/4 cup at a time--and smooth it in with a spatula or the back of a large spoon. You want most of the liquid to be absorbed by the cake, which will make it extremely moist.