Chocolate Cake with Fluffy Frosting

Preheat the oven to 350 degrees.To make the cake, combine the first 3 ingredients in a small bowl; let stand for 1 minute. Stir until smooth; set aside.Spoon flour into dry measuring cups; smooth with a knife. Combine the flour, baking powder, baking soda, and 1/4 teaspoon salt in a bowl, stirring with a whisk. Put the granulated sugar, buttermilk and butter in a large bowl; beat with a mixer on low speed until smooth. Add the chocolate mixture; beat on low speed for 1 minute. Add the flour mixture; beat on low speed until smooth.Combine the egg whites and tartar in a medium bowl. Using clean, dry beaters, beat the egg white mixture with a mixer on high speed until stiff peaks form (do not interrupt). Gently mix one-third of the egg white with the chocolate mixture. Add the remaining egg-white mixture. Spread the batter evenly over 2 (8-inch) round metal cake tins covered with baking spray. Bake at 350 ° C for 18 minutes or until a wooden stick inserted in the middle comes out with wet crumbs sticking to it. Refrigerate for 10 minutes in tins on a wire rack. Remove the cake from the pan and let it cool completely on a wire rack.To make the frosting, combine cream cheese, 2 tablespoons flour, vanilla, and 1/8 teaspoon salt in a large bowl. Beat with a mixer on medium speed for 2 minutes or until smooth. Add the powdered sugar; beat for 1 minute or until smooth. Gently stir in the marshmallow cream until smooth.Place 1 cake on a plate; spread with 1 cup icing, leaving a 1/2-inch-thick border around the edge. Top with the remaining cake. Spread the remaining icing on the top and sides of the cake. Cool the cake until ready to serve.