To prepare cake, combine first 3 ingredients in a small bowl; let stand 1 minute. Stir until smooth; set aside.
Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, and 1/4 teaspoon salt in a bowl, stirring with a whisk. Place granulated sugar, buttermilk, and oil in a large bowl; beat with a mixer at low speed until blended. Add chocolate mixture; beat at low speed 1 minute. Add flour mixture; beat at low speed just until combined.
Combine egg whites and cream of tartar in a medium bowl. Using clean, dry beaters, beat egg white mixture with a mixer at high speed until stiff peaks form (do not overbeat). Gently stir one-third of egg white mixture into chocolate mixture. Fold in remaining egg white mixture. Divide the batter evenly between 2 (8-inch) round metal cake pans coated with baking spray. Bake at 350° for 18 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool for 10 minutes in pans on a wire rack. Remove cake from pans, and cool completely on wire rack.
To prepare frosting, place cream cheeses, 2 tablespoons flour, vanilla, and 1/8 teaspoon salt in a large bowl. Beat with a mixer at medium speed for 2 minutes or until smooth. Add powdered sugar; beat 1 minute or until well combined. Gently stir in marshmallow creme just until combined.
Place 1 cake layer on a plate; spread with 1 cup frosting, leaving a 1/2-inch border around edge. Top with remaining cake layer. Spread remaining frosting over top and sides of cake. Refrigerate the cake until ready to serve.
The cream cheese based frosting seems better suited to some other flavor of cake but was otherwise pretty tasty (and fluffy). The cake itself wasn't too terribly special. It's not a very rich chocolatey flavor, but more of a light chocolate flavor.
Everything turned out fine and I did not experience the problems that others did with the cake not rising or being dry. Be sure to put as much icing in between the cake layers as the instructions say. There will be plenty more to ice the outside of the cake.
Overall it felt like a good bit of work for something that was fine but not spectacular.