Chocolate yule log cake with mocha cream filling, chocolate buttercream and meringue mushrooms

Separate the eggs. Whisk together the cake flour, three tablespoons of Dutch process cocoa powder, kosher salt, baking soda and baking powder, sifting if lumpy.

Whisk about a third of a cup of the sugar into the yolks until well mixed, without taking it to the ribbon stage.

Whisk in the optional instant coffee, two teaspoons of vanilla extract, then stream in five tablespoons of melted cooled butter and whisk smooth.

Whisk in the buttermilk, then whisk the dry ingredients into the wet until completely smooth and thick.

Whip four egg whites with a pinch of salt, starting on low; once frothy and opaque, cascade in the remaining third cup of sugar slowly so it dissolves, and beat to firm peaks that stand straight up.

Fold about a third of the whites into the chocolate batter to lighten it, then fold in the rest quickly, turning the bowl and bringing the bottom over the top without overmixing.

Grease only the bottom of a half sheet pan, press in parchment, scrape in the batter and spread it to the edges in a thin even layer with an offset spatula held nearly flat.

Bake at 350F, ten to fifteen minutes; check at ten. It is done when the surface is matte rather than shiny and springs back.

Dust a large clean kitchen towel with a thin layer of cocoa powder over an area the size of the pan.

Cut around the sides to release the cake, invert it hot onto the towel, peel off the parchment and roll it up tightly inside the towel to rest on its seam. Let it cool rolled so it keeps the curve and does not crack.

Pour about half of the cup of cream into a saucepan with three ounces of chopped milk chocolate and whisk over medium low until melted and smooth.

Off the heat, whisk in the rest of the cream, then the half cup of crème fraîche and a pinch of salt.

Set the pan in a bowl of ice water coming halfway up the sides and whisk until it thickens and cools.

Whip the filling to medium peaks, then add a little vanilla, a tablespoon and a half of brewed coffee and a teaspoon of instant coffee.

Unroll the cooled sponge slowly, spread the filling in a thin even layer filling the first curve and leaving a border at the far edge, then roll it back up using the towel and wrap it to hold the shape.

Chill at least two hours, or overnight, so the filling sets.

Whip 75 g of egg whites with a pinch of kosher salt and about an eighth of a teaspoon of cream of tartar until frothy and opaque.

Add the granulated sugar slowly, beat on medium high to a firm glossy peak, then add the powdered sugar all at once on low and beat back to firm peaks. Finish with a little vanilla paste.

Scrape the meringue into a piping bag fitted with a large round tip.

Pipe the stems first, holding the bag vertically just above the baking sheet, pressing and lifting; pipe at least a dozen.

Pipe the caps starting close to the sheet and lifting while decreasing pressure; flatten any peaks with a finger dipped in powdered sugar.

Hold a slotted spoon over each cap and sprinkle cocoa powder through a strainer for spots.

Dry the meringues at 200F, checking after 30 minutes and removing them as they are done.

Beat the two yolks and two whole eggs with a generous pinch of salt on medium until frothy, light and voluminous.

Combine the sugar and water in a small saucepan, pouring the water around the edges, bring to a boil, stop stirring once the sugar has dissolved, swirl the pan and cook to 238F.

Pour the syrup slowly down the side of the bowl into the beating eggs, avoiding the whisk, then beat on medium high until the bowl is cool to the touch.

Add the room temperature butter a couple of tablespoons at a time to make a smooth emulsion; if it goes soupy the butter melted, keep whipping and it will come back.

Add about two teaspoons of vanilla extract and the eight ounces of melted, cooled dark chocolate and whip on medium until smooth.

Trim one end of the chilled roll, cut off a length about a third of the total, then cut that length in half on a slight diagonal at about 20 degrees for the branches.

Set the large section on a platter and stick the two branches on with frosting, one to the side and one on top.

Coat the whole thing with buttercream and texture it like bark with a small offset spatula, using a dollop underneath anything that will not adhere.

Trim the untrimmed end with a serrated knife to reveal a clean cut side.

Grate a little chocolate over the top.

Stick a cap to a stem with a little buttercream, piercing the soft cap with the stem's point, and arrange the mushrooms in clusters.

Dust the finished log with powdered sugar.

Serve at close to room temperature so the buttercream is not hard; cut the branches first, then down the log.