Preheat oven to 325°F. Separate eggs, placing egg whites in a very large bowl and yolks in a small bowl; set aside. In a small saucepan heat the 3/4 cup milk over medium heat until simmering; remove from heat. Whisk in the cocoa powder (mixture will be thick); set aside to cool.
In a small bowl combine the flour and baking powder; set aside. Beat the egg whites and salt with an electric mixer on medium speed until frothy. Increase speed to medium-high and beat until soft peaks form (tips curl). Slowly add the granulated sugar, beating until stiff peaks form.
Add egg yolks to the beaten whites and beat just until combined. Add the flour and cooled chocolate mixture alternately to the egg mixture, beating well after each addition. Add the vanilla and beat just until combined.
Pour batter into an ungreased 13x9x2-inch baking pan, spreading evenly. Bake for 30 to 35 minutes or until a toothpick inserted near the center comes out clean. Transfer cake to a cooling rack and cool completely in the pan (about 1 hour).
In a medium saucepan combine the evaporated milk and espresso powder. Heat over medium heat, stirring constantly, until the espresso powder has dissolved. Remove from heat. Stir in the sweetened condensed milk and the 3/4 cup whipping cream.
Use the tines of a long fork or a wooden skewer to poke holes all over the top of the cake. Pour the espresso mixture evenly over the top of the cake.
Meanwhile, in a medium bowl beat the cream cheese with an electric mixer on medium speed until smooth; beat in powdered sugar. Add 1/4 cup of the whipping cream and beat until combined. Add remaining whipping cream and beat until soft peaks form.
Pipe or spread whipped cream mixture over cake and refrigerate for 3 to 24 hours, covering cake after topper is set. If desired, sprinkle with chocolate jimmies.