How to Make It
Preheat the oven to 325°. Butter a 9-inch square baking pan. In a medium bowl, whisk the flour with the baking powder. In a large bowl, using a handheld electric mixer, beat the egg whites at medium-high speed until stiff peaks form. In another large bowl, beat the egg yolks with the sugar at medium speed until pale and thick, about 3 minutes. Beat the egg whites and the dry ingredients into the egg yolk mixture until smooth.
Scrape the batter into the prepared pan and bake in the center of the oven for 30 minutes, until golden and a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 15 minutes, then invert onto a wire rack to cool completely.
In a 13-by-9-inch baking dish, whisk the condensed milk with the evaporated milk, heavy cream and rum. Cut the cake into 3-inch squares and add them to the baking dish, turning gently once or twice. Tilt the dish and spoon the liquid over the squares until well-soaked, about 5 minutes. Pour off all but a thin layer of the liquid and reserve for another use. Cover and refrigerate the cakes overnight.
Carefully transfer the squares to plates. Drizzle with the Dulce de Leche and serve.