Step 1

Preheat the oven to 350F. Prepare a 13" x 9" sheet cake pan by adding parchment paper.

Step 2

In a standing mixer, sift in the flour, baking powder, salt and sugar. Give it a whisk and set aside.

Step 3

In a large bowl, add egg whites, milk, vanilla, and sour cream. Whisk together.

Step 4

Attach a paddle attachment to the stand mixer. Add in room temperature butter in cubes or pieces.

Step 5

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.

Step 6

Pour the batter into the prepared sheet cake pan.

Step 7

Bake for 40 -50 minutes or until a toothpick inserted comes out clean.

Step 8

Let the cake cool to room temperature.

Step 9

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.

Step 10

In a stand mixer fitted with a whisk attachment, add heavy whipping cream, confectioners sugar.

Step 11

Beat on high until whipped.

Step 12

Add vanilla extract. Give it another whip on high.

Step 13

Fold in mascarpone by hand using a spatula. Switch to a whisk and beat.

Step 14

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.

Step 15

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.

Step 16

Use an ice cream scooper to add the frosting.

Step 17

Smooth with an offset spatula.

Step 18

Garnish each piece with a fan of strawberry slices and a sprig of mint.