Preheat the oven to 350°F. Butter and flour a 13x9 inch metal baking pan or spritz with baking spray.
Cake: In a medium-size bowl use a whisk to sift together all purpose flour, baking powder, salt and 1/8 tsp ground nutmeg. Set aside.
In the bowl of a stand mixer cream together butter, cup sugar, egg yolks, 2 tsp vanilla and 1 tsp rum extracts. Cream for 2-3 minutes.
To the bowl add the sifted dry ingredients alternately with the buttermilk. Stop to scrape the sides of the bowl as needed.
In a separate bowl whip the egg whites until stiff peaks form. Fold into the cake batter by hand. Pour batter evenly into the baking pan.
Bake for 28-30 minutes or until a toothpick inserted into the center comes back clean.
Rest for 20 minutes. Poke holes over the top of the cake with a fork or wooden skewer.
Tres Leches Filling: Whisk together the evaporated milk, sweetened condensed milk, 1/2 cup heavy cream, 1 tsp vanilla, 1 tsp rum extract and 1/8 tsp ground nutmeg.
Slowly pour 1/2 over the cake allowing it to seep into the holes then pour the remaining on top.
Cool to room temperature then cover and chill overnight.
Serve: Whip together the remaining 2 cup heavy cream with 1/4 cup granulated sugar and 1/2 tsp rum extract. Adjust sweetness taste.
Frost the top with the whipped cream and dust with nutmeg. Serve with seasonal berries.
Store leftovers chilled.