Tres Leches Cake

Preheat the oven to 350F. Grease a 9x13-inch baking dish.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, beat the egg yolks with 3/4 cup of the sugar until pale and thick, about 2 minutes. Beat in the whole milk and vanilla.
In a separate clean, dry bowl, beat the egg whites with a hand mixer on medium-high speed until foamy, then gradually add the remaining 1/4 cup sugar and continue beating until stiff, glossy peaks form -- the whites should stand straight up when the beaters are lifted.
Gently fold the flour mixture into the egg yolk mixture just until combined.
Fold about one-third of the beaten egg whites into the batter to lighten it, then gently fold in the rest in two more additions, being careful not to deflate the whites -- this is what gives the cake its light, spongy texture.
Pour the batter into the prepared dish and smooth the top.
Bake for 25-30 minutes, until the top is golden and a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan, about 45 minutes.
Once cool, use a fork to poke holes all over the top of the cake, spacing them about 1 inch apart -- this helps the milk soak in evenly.
In a bowl or large measuring cup, whisk together the evaporated milk, sweetened condensed milk, and 1/2 cup heavy cream.
Slowly pour the milk mixture evenly over the cooled cake a little at a time, letting it absorb before adding more, until it's all been poured on (the cake will look very wet, which is correct).
Cover and refrigerate for at least 3 hours, or overnight, so the cake fully absorbs the milk mixture.
Just before serving, whip the 1 1/2 cups cold heavy cream with the powdered sugar until soft peaks form, and spread it over the top of the soaked cake. Dust with ground cinnamon if desired, and slice to serve.

Fold the egg whites in very gently -- this cake's soft, absorbent texture depends on keeping as much air in the batter as possible.

Pour the milk mixture on slowly in stages rather than all at once, giving the cake time to soak each addition so it doesn't just pool and overflow the pan.