Serves 16 to 20
Position rack in bottom third of oven and preheat the oven to 350 degrees.
Lightly grease a 13-by-9-inch baking pan with 1 tablespoon shortening. Add 1 tablespoon flour to the greased pan and shake it around to coat entire pan with flour. Shake out excess flour. Set aside.
In the bowl of an electric mixer fitted with a whisk attachment, beat egg whites on medium speed until soft peaks form. Reduce speed to low and gradually add sugar with mixer running, beating until stiff peaks form.
Add egg yolks one at a time, beating well after each egg is added.
In a small mixing bowl, sift together flour and baking powder.
Add 1 teaspoon vanilla extract. Add flour mixture to batter in stages, alternating with whole milk, beginning and ending with flour. (Do this quickly so that batter does not lose its volume.) Pour batter into prepared pan and place in oven.
Bake for 25 to 30 minutes, until a toothpick inserted into center comes out clean.
Remove cake from oven and place on a wire rack to cool for 10 minutes.
In a blender, combine evaporated milk, sweetened condensed milk, and 2 cups heavy cream and blend on high for 45 seconds.
Reserve 1 1/2 cups milk mixture, cover, and refrigerate until ready to serve cake.
Use a toothpick to poke 8 to 10 holes in cake, and pour 1 1/2 cups milk mixture over warm cake.
When cake has soaked up most of the liquid, pour remaining half of milk mixture over cake, and cool to room temperature. Cover and refrigerate until well chilled, at least 4 hours or overnight.
When ready to serve, beat remaining cup heavy cream in electric mixer until soft peaks form. Add confectioners' sugar and remaining 1/4 teaspoon vanilla, and beat until stiff peaks form. Spread whipped cream over chilled cake.
Serve cake with reserved chilled milk sauce and garnish with cocoa powder.