Preheat your oven to 350°F (175°C). Lightly grease and flour a 9x13-inch baking dish.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, beat the egg whites until soft peaks form. Gradually add ½ cup sugar while continuing to beat until stiff peaks form. Set aside.

In another bowl, beat the egg yolks with the remaining ½ cup sugar until pale and thickened. Stir in the vanilla extract and ⅓ cup milk.

Gently fold the flour mixture into the egg yolk mixture. Then carefully fold in the beaten egg whites until just combined - be careful not to deflate the whites.

Pour the batter into the prepared baking dish and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool for 10 minutes.

While the cake is cooling, whisk together the condensed milk, evaporated milk, heavy cream, and vanilla in a bowl.

Using a fork, poke holes all over the cake. Slowly pour the milk mixture over the cake, allowing it to soak in. Cover and refrigerate for at least 4 hours or overnight.

Just before serving, whip the heavy cream with powdered sugar and vanilla until stiff peaks form.

Spread the whipped cream over the soaked cake. Dust with ground cinnamon if desired. Cut into squares and serve.