3-4tablespoonsheavy cream, half and half, or milk(1 1/2 to 2 ounces; 42 to 56 grams)
3ouncesunsweetened chocolate squares, melted
Adjust oven rack to middle position. Preheat oven to 350 degrees F. Spray an 18x13-inch baking pan with nonstick cooking spray, line the bottom of the pan with a piece of parchment paper, and then lightly spray the paper.
Whisk together white rice flour, sweet rice flour, cocoa powder, baking powder, salt, and xanthan gum in a medium bowl. Set aside.
Whip eggs in a stand mixer fitted with the whisk attachment until thick, about one minute. After one minute, add the granulated sugar in a slow and steady steam. Continue to whip the eggs until they are very thick, about 6 minutes. (This takes about 4 minutes on a stand mixer and up to 8 minutes with a handheld mixer.)
Reduce mixer to low. Add flour mixture, one tablespoon at a time. Mix until just combined. Spread batter evenly into the prepared pan.
Bake until the cake is aromatic and springs back to the touch, about 14 minutes.
While the cake bakes, place a large, clean kitchen towel on your counter. Dust it generously with powdered sugar.
Remove pan from the oven and immediately run a knife around the edges of the pan. Then, carefully, flip the cake out onto the prepared towel. Slowly pull the piece of parchment off the cake.
Starting from the long end, roll the cake into a snug log. Allow the cake to cool for about one hour.
While the cake cools, prepare the filling: In a medium bowl, cream butter until light and fluffy. (Use high speed on a handheld mixer or medium-high speed on a stand mixer.) Add the powdered sugar and beat until combined. Add the marshmallow crème and vanilla extract. Mix until fluffy.
Slowly unroll the cake. Spread filling over the cake, leaving about a 1/2-inch border around the cake. Re-roll the cake (without the towel this time). Using a serrated knife, trim the edges. Discard or eat as a cook's treat. Cut off two slices at a diagonal, about 2 to 3-inches each. Get creative with this! I like them to be different sizes. Set slices aside.
Transfer log to serving platter.
Prepare the frosting: cream together butter and powdered sugar in a large bowl. Add two tablespoons heavy cream. In a slow and steady stream, add the melted chocolate. Beat until smooth. Add one tablespoon heavy cream and vanilla extract. If icing is too thick, add remaining heavy cream. Use at once.
Frost cake with prepared chocolate frosting. Place icing on each slice and stick it to the cake where you'd like. You can place the pieces on either side of the log. Or stick one on top and the other on the side. Frost the slices. Serve.