Preheat oven to 350°F. Line two 9-inch square cake pans with 2-inch-high sides with foil, extending foil over sides. Butter and flour foil. Using electric mixer, cream 3 sticks of butter and 1 1/2 cups dark brown sugar and 1 cup white sugar in large bowl until fluffy. Beat in melted unsweetened chocolate, 1/4 cup cream and 2 tsp vanilla. Add 6 eggs 1 at a time, beating well after each addition. Sift flour, cocoa, baking soda and salt into medium bowl. Stir dry ingredients into batter, which will be very thick. Divide batter between prepared pans; spread evenly. Bake until tester inserted into centers comes out clean, about 20 minutes. Remove from oven. Sprinkle 1 1/2 cups marshmallows and 1 cup pecans over each cake. Bake just until marshmallows begin to melt, about 6 minutes. Transfer to racks. Cool cakes completely in pans.
Combine 1 cup brown sugar and 1 cup whipping cream in heavy medium saucepan. Stir over medium heat until sugar dissolves. Cook without stirring until candy thermometer registers 220°F., about 8 minutes. Remove from heat and whisk in 1 stick of butter, 5 oz. of unsweetened chocolate, 2 oz. of bittersweet chocolate, 2 tbsp dark corn syrup and 1 tsp vanilla extract. Whisk until all chocolate melts and frosting is smooth. Refrigerate frosting until spreadable, about 1 hour 15 minutes. Using foil as aid, lift cakes from pans. Remove foil. Place 1 cake layer on plate, marshmallow side up. Spread 1 1/2 cups frosting over. Top with second cake layer, marshmallow side up. Spread remaining frosting in waves on sides (not top) of cake. (Can be prepared 1 day ahead. Cover with cake dome and chill. Bring to room temperature before serving.)