For the Cake:
Preheat the oven to 350F. Prepare a 13" x 9" sheet cake pan by adding parchment paper. Set aside. 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. On medium speed, beat half of the wet mixture into the crumby dry mixture. Scrape the bowl down and add the remaining wet mixture. Beat until well combined.
Pour the batter into the prepared sheet cake pan.
Bake for 45 - 50 minutes or until a toothpick inserted comes out clean.
Let the cake cool to room temperature. Set aside.
For the Tres Leche syrup:
Add whole milk, sweetened condensed milk and evaporated milk into a large bowl, whisk together and set aside. If you're choosing to add dark rum now it’s the time.
For the Frosting:
1. In a stand mixer fitted with a whisk attachment, add heavy whipping cream, confectioners sugar.
Beat on high until whipped.
Add vanilla extract. Mix on high speed until stiff peaks form.
For the Assembly:
Polk holes all over the cake. Slowly drizzle the tres leches on the cake.
Add frosting with a measuring cup or an ice cream scooper.
Smooth the surface with an offset spatula.
Place ground cinnamon in a sieve and lightly dust the top of the cake.
Garnish cake each cake piece with a fan of strawberry slices.