3 large eggs, separated
1 2/3 cups granulated sugar
1 1/3 cups all-purpose flour
1 3/4 teaspoon baking powder
1/4 cup plus 1 tablespoon whole milk
1 teaspoon pure vanilla extract
Tres Leches Soaking Liquid
1 cup heavy cream
1 (12-oz) can evaporated milk
3/4 cup sweetened condensed milk (half a 12-oz can)
1 cup firmly packed dark brown sugar
1 tablespoon ground cinnamon or canela
1/2 teaspoon freshly grated nutmeg
1/4 teaspoon kosher salt
1/4 cup dark rum or mezcal (optional)
Place a rack in the middle position and preheat the oven to 325°F.
To make the cake, whip the egg whites on high speed in a clean, dry bowl while slowly adding the granulated sugar. Continue whipping the mixture until it is fluffy, shiny, and firm but not dry. With the mixer still running, slowly add the yolks.
Sift the flour with the baking powder and fold into the egg mixture. Whisk in the milk and vanilla.
Lightly coat a 9 by 9-inch square cake pan or casserole with cooking spray and pour in the batter. Bake until the center of the cake springs back to the touch and a wooden skewer inserted into the middle comes out clean, 40 to 45 minutes, rotating the pan 180 degrees about halfway through the baking time. Cool the cake in the pan on wire rack for 10 minutes.
To make the soaking liquid, while the cake is still warm, whisk together all the ingredients for the soaking liquid until the sugar dissolves. Poke the top of the cake 8 to 10 times with a fork to make evenly spaced holes, then pour the soaking liquid over the warm cake to completely saturate it. Leave the cake to soak at room temperature until the liquid is fully absorbed and the cake has cooled to room temperature. Cover the pan tightly with plastic wrap and refrigerate for at least 2 hours and up to 3 days.
Fifteen to 20 minutes before serving, take the cake out of the refrigerator and leave it to sit and soften at room temperature. Cut the cake into individual rounds (see photo) or squares and serve.