Yellow cake mix with pudding (plus the ingredients to make it)
1 can sweetened condensed milk (Eagle brand)
2 cans evaporated milk
strawberries, sliced (optional)
Make the yellow cake according to directions on the box. Let it cool, then use a fork to poke holes throughout the top. Mix the sweetened condensed milk and evaporated milk in separate bowl. Pour milk mixture over the cake (make sure to get around the edges) and put in the fridge for several hours or overnight. The milks should be absorbed and the cake should be moist and spongy. Frost with the whipped cream just before serving. Garnish with strawberry slices, if desired.
I had a friend who was on an eternal quest to find a recipe for Tres Leches that was just as good as Cafe Rio's. He said this was a close contender. I hear that the recipe found here is even better, but it has WAY too many ingredients for me. So, I'll let you be the judge.