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.