Step 1

Reduce the oven temperature to 200F. Line a baking sheet with parchment paper.

Step 2

In a large mixing bowl, combine the egg whites and cream of tartar. Beat on medium speed until frothy and doubled in volume. With the mixer running, slowly sprinkle in the sugar. Once all of the sugar is added, continue beating until stiff and glossy peaks form, about 5 minutes. Place the meringue in a piping bag fitting with a large round tip. (I used a Wilton 2A tip.)

Step 3

Pipe half of the meringue into stems by placing the piping tip on the parchment and squeezing the piping bag while slowly moving the tip up, creating a column of meringue, about 1-inch tall. Pipe round tops about two times wider than your stems by placing the piping tip about ½-inch above the parchment and squeezing the bag until the desired size. Dust the tops and stems very lightly with cocoa powder, if desired.

Step 4

Bake for about 2 hours or until the meringues easily release from the paper. Turn the oven off and let cool completely in the oven.

Step 5

Once cool, use a sharp knife to remove any points on the tops of the stems if needed. Dip the flat bottom of the mushroom tops in melted chocolate and place on top of a stem. Set aside until set, about 1 hour.

Step 6

Preheat the oven to 350F. Lightly spray a 12x17-inch rimmed baking pan with baking spray. Fully line with parchment paper and lightly spray the paper.

Step 7

In a medium bowl, sift together the flour, cocoa powder, and salt.

Step 8

In a large mixing bowl, add the egg yolks. Beat on medium speed until about doubled in volume and velvety. Add 3/4 cup sugar and continue beating until very pale and fluffy, about 2 minutes. Sprinkle in the flour mixture and beat just until combined. (Mixture will get very thick.)

Step 9

In another large bowl with clean beaters, beat the egg whites on medium speed until light and frothy. With the mixer running, slowly add the remaining 1/4 cup of sugar. Continue beating until stiff, about 4 minutes. Fold the beaten egg whites into the yolk mixture in three batches, folding just until no white streaks remain. Spread the batter in an even layer in the prepared pan.

Step 10

Bake for 20 minutes or until the cake springs back when gently pressed in the center. Dust a clean tea towel with confectioners’ sugar. Immediately invert the hot cake onto the towel. Remove the pan and carefully peel off the parchment paper. Starting at a long end, somewhat tightly roll up the cake with the tea towel. Place on the counter seam side down to keep it from unrolling, and cool completely, about 1 hour.

Step 11

In a large bowl, beat the heavy cream on medium speed until thickened. Add the sugar and vanilla. Continue beating until stiff. (Be careful not to go too far and curdle it.)

Step 12

Unroll the cooled cake, leaving it on the tea towel. Spread the whipped cream over the cake leaving a 1-inch border. Using the tea towel to help, roll the cake up again, starting from the same long end. If any excess cream squeezes out the seam, scrape it off, and place the cake roll seam-side down on a tray. Place in the fridge.

Step 13

Place the butter and salt in a mixing bowl and beat on medium speed until smooth. Add the cocoa powder and beat until well combined, stopping to scrape down the bowl a few times, about 2 minutes. With the mixer on low, beat in the confectioners’ sugar and vanilla. Slowly stream in the heavy cream while beating. Increase mixer speed to medium and beat until fluffy and easily spreadable, about 2 minutes.

Step 14

Remove the cake log from the fridge. Cutting at a diagonal, cut about two inches off of one end. Place the log on a serving plate. Spread a few tablespoons of frosting onto the end of the cut-off and stick to the side of the log. (The diagonal cut should be facing out.) Spread the remaining frosting all over the cake roll, leaving the ends exposed.

Step 15

Run fork tines through frosting in one direction to give the appearance of bark. Trim the un-cut end of the log to expose a perfect swirl. Refrigerate the cake until ready to serve.

Step 16

Just before serving, decorate the log with a few of the meringue mushrooms, sugared cranberry and rosemary, and dust lightly with confectioners’ sugar.