Preheat the oven to 350F. Prepare a 13" x 9" sheet cake pan by adding parchment paper.
In a standing mixer, sift in the flour, baking powder, salt and sugar. Give it a whisk and set aside.
In a large bowl, add egg whites, milk, vanilla, and sour cream. Whisk together.
Attach a paddle attachment to the stand mixer. Add in room temperature butter in cubes or pieces.
On medium speed, beat for ½ of the wet mixture into the crumby dry mixture. Scrape the bowl down and add the remaining wet mixture. Beat until combined.
Pour the batter into the prepared sheet cake pan.
Bake for 40 -50 minutes or until a toothpick inserted comes out clean.
Let the cake cool to room temperature.
For the Syrup:
Add the three milks into a large bowl, whisk together and set aside. You can add more milk to cut the sweetness if desired. If you're adding dark rum go ahead and mix it in now. Start with 1/4 cup and add more to taste.
For the Frosting:
In a stand mixer fitted with a whisk attachment, add heavy whipping cream, confectioners sugar.
Beat on high until whipped.
Add vanilla extract. Give it another whip on high.
Fold in mascarpone by hand using a spatula. Switch to a whisk and beat.
For the Assembly:
I trimmed the sides and top of the cake. You don't have to do this, but I wanted a nice light edge and it makes the cake even more absorbent.
Slowly drizzle on the tres leches on the cake. You can polk holes all over the cake to make more room for the sweet milk mixture if desired.
Use an ice cream scooper to add the frosting.
Smooth with an offset spatula.
Garnish each piece with a fan of strawberry slices and a sprig of mint.
Add 1/2 cup of rum to the tres leches mixture, soak the cake and enjoy! Doing this brings you closer to the original recipe, which had alchohol!
Trimming the cake is TOTALLY optional. I did it because this is a giant sheet cake that has to bake for a long time. Those edges get caramelized and take away from the visual for me but you might enjoy them!
You can substitute whole milk yogurt for the sour cream if desired, I do it all the time and can’t really tell the difference.
The mascarpone is optional but If you can't get ahold of it and still want to have a similar effect try mixing the following room temperature ingredients together for a reasonable facsimile. 8oz cream cheese, 1/4 cup heavy cream, 2tbs unsalted butter.