First, preheat the oven to 350°. Crack the eggs and separate the whites from the yolks. Place the whites in the fridge for later.
In the bowl with the yolks, add the sugar and beat it with a hand mixer until it's smooth. Add flour, baking powder, vanilla, and milk. Mix with a hand mixer until combined.
Remove the whites from the refrigerator. Add salt and beat them with the hand mixer on high for 2-3 minutes until soft peaks form. With a rubber spatula, gently fold the egg whites into the cake batter.
Grease the bottom of an 8×8 or equivalent baking dish with the butter. Pour the cake batter into the dish and bake at 350 for 50 minutes, until a toothpick inserted comes out clean.
Let the cake cool for 10-20 minutes.
For the Tres Leches
While the cake is cooling, mix together the "tres leches" ingredients in a separate bowl. Poke holes with a straw or make small slits with a knife throughout the cake.
If serving right away, pour 1/2 of the milk mixture over the holes in the cake and let it rest for 30-60 minutes. When serving, cut a piece of cake and serve with a moat of the remaining milk around the cake on the plate.
If serving later that day or the next day, pour 1/3 of the milk mixture over the cake. Cover and refrigerate. Refrigerate the remaining milk mixture separately in an air-tight covered bowl. When you're ready to serve, remove the cake from the fridge, slice pieces, and serve by pouring more of the milk mixture over each piece.
Top with a caramel drizzle and sliced strawberries.
Notes
The cake should be thick, so we'd recommend using an 8×11 rectangular baking dish. You want the dish to be at least 2/3 full.
If you’re serving six slices, you’ll want about 1/4 cup of the milk mixture per slice. Or if you cut smaller slices, serve with a little less of the milk mixture.