Preheat oven to 350°F. Grease a rimmed half-sheet pan (18-inch by 13-inch), line with parchment paper and grease the paper.
In a medium bowl, whisk together flour, cocoa powder and baking powder.
In the bowl of a stand mixer fitted with a whisk attachment, beat egg whites and salt on medium speed until foamy. With the mixer running, gradually add ½ cup sugar, a tablespoon at a time, until medium-stiff peaks form. Transfer whites to a bowl.
Add egg yolks, remaining ¼ cup sugar, and vanilla to stand mixer bowl. Beat on medium speed until creamy and lightened, about 2 minutes. Mix in the melted butter until just combined. Fold in the dry ingredients until just combined.
Fold a third of the egg whites into batter to lighten. Gently fold in the rest of the egg whites in 2 parts, stopping when just combined.
Pour batter into prepared pan and lightly smooth the top with an offset spatula. Bake 10-12 minutes, or until the cake springs back when lightly pressed. Meanwhile, sift a generous layer of cocoa powder on a clean dish towel, at least 12-inch x 17-inch large.
Remove cake from oven and run a knife around the edge of the pan to loosen it. Quickly and carefully invert it onto the prepared dish towel. Peel off the parchment paper. Gently roll the cake up with the towel, starting with a short edge. Let it cool completely in the towel.
To make filling, combine marshmallow fluff, powdered sugar, salt and cream in the bowl of a stand mixer fitted with a whisk attachment. Whip until medium peaks form, about 3 minutes.
Carefully unroll cake and spread filling over the surface, leaving 1/2-inch border around the cake. Gently reroll the cake over the filling, without the towel, to create a tight log. Place it seam side down on a large platter, cover loosely, and refrigerate at least 1 hour.
Diagonally slice a 4-inch piece off the roll and arrange it like a branch on the larger log. Spread whipped chocolate ganache (recipe below) all over the cake and use the back of a spoon to create the texture of bark along the surface. Chill until ready to serve.
Decorate with sugared herbs, if desired.
Place chocolate in a medium heatproof bowl. Heat heavy cream in a small saucepan until just beginning to bubble at the edges. Pour over chocolate and let sit 4-5 minutes. Whisk until smooth. Let it cool to room temperature until set but still soft, about 2 hours.
Add vanilla and salt and beat with a handheld mixer until light and fluffy. Use immediately.