Martha takes tres leches cake to the next level by adding yet another milk. The vanilla-flecked batter is enriched with milk powder, whole milk, condensed milk, and evaporated milk. Finish with whipped cream and your favorite fruit.
Preheat oven to 325 degrees. Lightly butter a 13-by-9-inch baking dish. In a medium bowl, whisk together flour, milk powder, 1/2 cup sugar, baking powder, and salt. In a separate bowl, whisk together 1/2 cup whole milk, oil, egg yolks, and vanilla seeds. Whisk egg-yolk mixture into flour mixture.
In the bowl of a mixer fitted with the whisk attachment, beat egg whites on high speed until frothy. Add cream of tartar and beat until soft peaks form. Gradually add remaining 1/2 cup sugar, beating until stiff, glossy peaks form, about 5 minutes. Using a rubber spatula, fold one-third of egg-white mixture into batter. Gently but thoroughly fold in remaining egg-white mixture. Transfer to prepared dish. Bake until top springs back when lightly touched, about 40 minutes.
In a medium bowl, whisk together condensed milk, evaporated milk, and remaining 1 cup whole milk. Poke warm cake all over with a wooden skewer or toothpick, then pour milk mixture over top. Let cool to room temperature, about 1 hour.
Whisk cream and sugar to medium peaks. Spread whipped cream over cake and serve with fruit, if desired.