Preparation Position rack in center of oven and preheat to 350°F. Butter two 9-inch-diameter cake pans with 1 1/2-inch-high sides. Line bottom of pans with parchment; butter parchment. Dust pans with flour; tap out excess. Stir chocolate in top of double boiler over simmering water until melted and smooth. Remove from over water. Cool just to room temperature. Pour yogurt into 2-cup measuring cup. Add enough water to measure 1 1/2 cups; whisk to blend. Whisk in vanilla. Combine flour, sugar, butter, eggs, baking soda, baking powder and salt in large bowl. Add yogurt mixture and melted chocolate. Using electric mixer, beat until smooth, occasionally scraping bowl with rubber spatula, about 3 minutes. Divide batter between prepared pans. Bake cakes until tester inserted into center comes out clean, about 25 minutes. Cool cakes in pans on racks 10 minutes. Run small knife around sides of pans to loosen cakes. Invert onto racks; peel off parchment. Cool completely. Place 1 cake layer, flat side up, on platter. Spread 2/3 cup frosting over. Top with second cake layer, flat side down. Spread remaining frosting over top and sides of cake. Press marshmallows around top edge in 1-inch band and 1 inch down sides from top edge of cake. Sprinkle walnuts over center of cake. (Can be prepared 1 day ahead. Cover with cake dome and store at room temperature.)
Preparation Position rack in center of oven and preheat to 350°F. Butter two 9-inch-diameter cake pans with 1 1/2-inch-high sides. Line bottom of pans with parchment; butter parchment. Dust pans with flour; tap out excess. Stir chocolate in top of double boiler over simmering water until melted and smooth. Remove from over water. Cool just to room temperature. Pour yogurt into 2-cup measuring cup. Add enough water to measure 1 1/2 cups; whisk to blend. Whisk in vanilla. Combine flour, sugar, butter, eggs, baking soda, baking powder and salt in large bowl. Add yogurt mixture and melted chocolate. Using electric mixer, beat until smooth, occasionally scraping bowl with rubber spatula, about 3 minutes. Divide batter between prepared pans. Bake cakes until tester inserted into center comes out clean, about 25 minutes. Cool cakes in pans on racks 10 minutes. Run small knife around sides of pans to loosen cakes. Invert onto racks; peel off parchment. Cool completely. Place 1 cake layer, flat side up, on platter. Spread 2/3 cup frosting over. Top with second cake layer, flat side down. Spread remaining frosting over top and sides of cake. Press marshmallows around top edge in 1-inch band and 1 inch down sides from top edge of cake. Sprinkle walnuts over center of cake. (Can be prepared 1 day ahead. Cover with cake dome and store at room temperature.)