Preheat the oven to 350F.
Sift the flour.
With an electric mixer, over medium speed, beat the egg whites, baking soda and salt until soft peaks.
Lower the speed of the mixer and add the egg yolks and sugar. Continue mixing until smooth, about 30 seconds.
Add the butter, folding it in (I used a spatula). Then, mix in the flour in three or four batches, mixing by hand (carefully, so as not to break the precious air bubbles that will add volume to the cupcakes) until the mixture is well blended.
Pour into an eight or 10-inch bundt pan.
Bake in a 350F oven for 35 to 40 minutes or until a skewer poked at the thickest part of the cake comes out clean.
While the cake is still hot, poke holes all over it. Use a bamboo skewer or a large fork. When I made the tres leches cupcakes, the holes were too small and it took forever for the three-milk mixture to sink and get absorbed. So, make sure that you have larger holes and that the holes deep enough to reach the bottom of the pan. And poke a lot of holes. Make them as close as one centimeter apart.
Mix together the evaporated filled milk, cream and sweetened condensed milk. Pour the mixture over the cake, little by little, giving the thick liquid a chance to be absorbed before proceeding to pour some more. Just keep on doing it until all the liquid has been poured.
Now, the hardest part. Waiting. It is best to chill the cake for a couple of hours. I insisted that we wait. And it was worth the stressful waiting.