I have tried many Tres Leche cakes and didn't understand the hype on them...until I tried one at a new local Mexican restaurant...it was to die for! Talked to the cook, he shared a few ingredients and techniques they use...and true to my form, went home and researched and re-created this recipe. I have had raves from everyone who has tried it...NOW I understand the hype!
Heat the oven to 325 degrees with the rack in the center of the oven. Coat a 13 x 9 baking dish with the butter, set aside.
Mix the flour well in a small bowl to aerate and break up any lumps, or sift in a sifter if you have one. Set aside.
Separate the eggs, putting the yolks in the bowl of a stand mixer, keep the whites separate in a very clean, medium sized bowl. Add the cake sugar to the yolks in the mixing bowl and using the paddle attachment, beat on high speed until pale yellow (3-5 minutes). Transfer this mixture to a large, clean bowl and set aside. Thoroughly clean and dry the mixer bowl.
Into the cleaned mixer bowl, add the egg whites and using the whisk attachment, beat in high speed until stiff peaks form, about 2 minutes.
With a rubber spatula, gently fold about 1/2 of the egg white mixture into the egg yolk mixture until just combined. Add the the remaining whites mixture to the yolk mixture, folding it in gently (must be by hand, no mixer for this step). Sprinkle the flour over the top on the mixture and gently fold in, just until combined (DO NOT OVERMIX or mix with a mixer!)
While this is baking, place all 3 milks and the rum, in a large bowl and whisk until combined. Set aside.
Pour the batter into the prepared buttered baking dish and bake until the cake is puffed and golden brown. The edges will pull away from the sides of the baking pan. 20 - 25 minutes.
Remove the cake and cool slightly (10 min), then cool on a wire rack for 20 minutes, then return to the cooled baking pan. Using a bamboo skewer, poke holes all over the cake (lots of them, don't be shy!) and cool the cake another 30 minutes.
Pour the milk mixture over the cake, slowly letting it all in (it will take all of it). Let set for 30 minutes then cover tightly with plastic wrap and put into the fridge. Let it set at least 8 hrs, I recommend overnight, the milks will absorb into the cake 95 - 100% by then. This is what you want.
When ready to serve the cake, toast the coconut. In a large pan, spread the coconut and cook over medium heat, stirring constantly until lightly browned and fragrant. It burns easily, you may have to turn down the heat. Remove from pan into a small bowl. Set aside. In your stand mixer, pour the whipping cream and add the powdered sugar. Beat with the whisk attachment until peaks form. Add the sugar (for the whipped cream), rum and vanilla, beat again for 30 more seconds or until the flavors are incorporated.
Frost the cake with the flavored whipping cream and sprinkle with the toasted coconut. Keep this refrigerated, if you have any left to refrigerate!