Make the Cake Batter: Preheat oven to 350 degrees Fahrenheit. Grease and flour 13 x 9 cake pan and set aside. Sift flour into mixing bowl then whisk together with sprinkles, baking powder, and salt.
1 ½ cup all-purpose flour, ¾ cup rainbow sprinkles (jimmies), 1 ½ teaspoon baking powder, ¾ teaspoon fine sea salt
Using stand mixer fitted with paddle attachment, beat the egg yolks and sugar together until they develop a pale yellow color thick and fluffy (about 4 minutes). Mix in the vanilla paste or extract, rum, and ½ cup of the whole milk then fold in the dry flour mixture by hand until just combined and no dry pockets remain. This batter will be thick!
6 eggs, 1 cup granulated sugar, 1 ½ teaspoon vanilla bean paste, 1 teaspoon dark rum, 1 cup whole milk
In another bowl, use the whisk attachment to beat the egg whites until stiff peaks form (about 4-5 minutes).
Gently fold the egg whites into the batter by hand, mixing until smooth with no lumps of egg white. Go slowly and gently with this process to avoid deflating the egg whites as they are key to the cake’s signature texture.
Bake the Cake: Transfer batter into prepared pan and bake for 20 minutes or until top is golden brown and a toothpick inserted into the center comes out clean. Cool to room temp in baking pan (about 30 min).*The cake should have a bit of a dome when it comes out of the oven, but as it cools, it will fall flat and pull away from the edges of the pan. Make the Tres Leches: Once cake is fully cooled, stir together the remaining whole milk, sweetened condensed milk, and evaporated milk. Use a toothpick or fork to poke holes all over the top of the cake, then pour the milk mixture over top. Place cake in refrigerator to chill for 2-3 hours or up to overnight.
14 ounces sweetened condensed milk, 12 ounces evaporated milk
Decorate the Cake: Once cake is chilled, use stand mixer fitted with whisk attachment to whip together the heavy cream, powdered sugar, vanilla, and salt until soft, fluffy peaks form (about five minutes).
1 ½ cup heavy cream, ¼ cup powdered sugar, 1 teaspoon vanilla bean paste, ⅛ teaspoon fine sea salt
Spread whipped cream over top of cake then sprinkle with more sprinkles and serve.