What could make tres leches better? Fruity Pebbles, of course!
In a medium-sized bowl, combine the Fruity Pebbles, evaporated milk, and whole milk. Stir well, cover with plastic wrap, and set aside for an hour. Give the mixture a couple of stirs while it's soaking. After the mixture has soaked, strain out the Fruity Pebbles with a mesh strainer, pushing gently on the cereal to remove any excess liquid but being careful not to make the cereal too mushy. Set aside the cereal in a separate bowl. Add one can of sweetened condensed milk and lemon zest to the strained milk, cover, and place in the fridge while you make the cake.
Preheat oven to 350 F. Liberally spray a 9x13 pan with non-stick cooking spray and set aside.
In a large bowl, sift together the flour, baking powder, and salt and set aside. Stir together the soaked cereal and the 1/3 cup of whole milk and set aside.
In the bowl of a stand mixer fitted with a whisk attachment, mix together 3/4 cup of the sugar and egg yolks until the yolks are pale. Combine the milk, cereal mixture, and vanilla and stir gently until combined. Fold mixture into the dry ingredients until combined and no dry flour remains.
Wash and dry the mixer bowl and whisk, then add egg whites to the mixing bowl. Beat on high speed until the whites are foamy, then add the remaining 1/4 cup sugar. Continue beating until whites are stiff but not dry.
Fold the egg white mixture into the batter until just combined. Pour the batter into the pan and bake for 30-35 minutes, or until a toothpick inserted into the middle comes out clean and cake is no longer jiggly.
At this point, you can turn the cake out onto a rimmed platter or you can turn it upside down and put it back in the same cake pan. Let the cake cool completely.
After the cake has cooled, poke the cake all over with a fork. Pour the soaking liquid over the cake and let the liquid soak into the cake for about 30 minutes. After that, you can place the cake back in the fridge until you plan to eat it, or if you're planning on eating it right away, continue on making the whipped cream.
When you're ready to eat the cake, whip together the cream and sugar on high speed until the cream is stiff. Frost the cake with the cream and top with additional Fruity Pebbles, if you'd like. Cut into slices and enjoy!