Step 1

Preheat the oven to 325 degrees F, lightly mist the sides of a 10x15-inch jelly roll pan with non-stick spray, and line it with parchment.

Step 2

Place the eggs, sugar, vanilla, and salt in a large bowl on whip on high speed for a full 5 minutes. (The mixture should be very pale and fluffy, and tripled in volume.)

Step 3

Fold in the flour, cocoa, and baking powder.

Step 4

Whisk the sour cream, melted butter, and milk together, then fold the mixture into the batter.

Step 5

Transfer the batter to the prepared pan, smoothing it out to the corners in an even layer.

Step 6

Bake for 22 to 28 minutes, or until the cake is fully set and springy in the center but not dried out.

Step 7

While the cake is still hot, dust it generously with powdered sugar and lay a clean kitchen towel on top.

Step 8

Starting at one of the short sides of the cake, carefully roll it into a tight log and allow it to cool completely before filling, frosting, and garnishing.

Step 9

Place the powdered sugar, cocoa, cream cheese, and vanilla in a large mixing bowl, and whip together until smooth.

Step 10

Scrape the bottom and sides of the bowl with a silicone spatula, and slowly drizzle the cream in while continuing to whip on medium speed. (Stop frequently to scrape the bottom and sides of the bowl, to avoid lumps.)

Step 11

When all the cream has been added, turn the mixer up to high speed and whip until the filling holds stiff peaks.

Step 12

When the cake has completely cooled, gently unroll it and spread the filling on in an even layer.

Step 13

Carefully roll the cake back up into a log, frost with whipped ganache, and garnish.

Step 14

Warm the cream in the microwave (or in a small pot on the stove) until just beginning to simmer, then pour it over the chocolate and allow it to stand for 5 minutes.

Step 15

Whisk the ganache together until smooth, then allow it to cool until firm but still soft.

Step 16

Whip the ganache on high speed until fluffy (about 3 to 4 minutes).

Step 17

Use a small spatula to frost the yule log cake with the whipped ganache, dust with powdered sugar, and garnish.

Step 18

Preheat the oven to 200 degrees F and line a baking sheet with parchment.

Step 19

Place the egg whites in a large, clean mixing bowl, and add the cream of tartar and salt.

Step 20

Whip the whites on medium speed until foamy and just beginning to turn white.

Step 21

While continuing to whip, add in the sugar very slowly (about 1/4 teaspoon at a time).

Step 22

When all the sugar has been added, turn the mixer up to high speed and whip until the meringue is glossy and very stiff.

Step 23

Transfer the meringue to a clean piping bag fitted with a jumbo round tip, and pipe half the meringue into small dome shapes; half the meringue into tall stems.

Step 24

Bake the meringues for 2 to 2 1/2 hours, or until light and crisp.

Step 25

When the meringues have fully cooled, trim the points away from the tops of the stems with a sharp paring knife.

Step 26

Dip the bottoms of the caps in melted chocolate and adhere them to the tops of the stems, then dust the tops of the mushrooms with cocoa powder for garnish.

Step 27

Dust the cake roll with powdered sugar, then garnish with meringue mushrooms, fresh greens, and sugared cranberries.