Sponge Cake

1. Preheat oven to 220°C (425°F).  Line a 9 × 13 inch jelly roll pan with parchment and grease well.
2. Wipe bowl with lemon juice or vinegar. Whisk egg whites with salt + cream of tartar until foamy. Gradually add 50 g sugar; beat to soft peaks.
3. In a clean bowl, beat yolks with 50 g sugar, vanilla, and salt until pale and creamy, 5–7 min.
4. Sift flour + cocoa; fold into yolk mixture.
5. Fold egg whites gently into yolk mixture. Batter should be airy and pourable.
6. Spread in prepared pan, smoothing evenly.
7. Bake 10–12 min, until springy. Cool with parchment underneath.

Mascarpone Filling

1. Chill mixing bowl.
2. Beat mascarpone, cream, sugar, vanilla, almond extract, and salt until soft peaks form, 1–2 min.

Buttercream Mushrooms

1. Beat shortening + butter until smooth.
2. Add sugar, meringue powder, vanilla, cream, and salt. Beat 2–4 min.
3. Divide frosting: half white, half tinted red. Smooth and transfer to piping bags fitted with open round tips.
4. Pipe stems + caps on wax paper. Add sprinkles to caps. Freeze as you continue working through the recipe.
5. Once pieces are firm, assemble mushrooms by attaching stems + caps with buttercream.

Chocolate Buttercream

1. Beat butter, sugar, cocoa, cream, vanilla, and salt until fluffy, 5 min. Adjust cream if needed.
2. Smooth and transfer to piping bag with textured tip (like Wilton tip 1M).

Chocolate Bark

1. Melt ⅔ chocolate over double boiler to 113–120°F.
2. Stir in remaining ⅓ until cooled to 82°F.
3. Briefly rewarm to 88–90°F. Spread thinly on parchment, sprinkle pistachios + salt, and roll into parchment paper to set.
4. Once set, break chocolate into shards for attaching to log.

Assemble and Decorate Log

1. Spread mascarpone over sponge. If desired, sprinkle pistachios and mini chips across filling for textured insides.
2. Roll tightly with parchment. Chill 15 min.
3. Trim ends. Attach a branch if desired, and place log on serving board or platter for display.
4. Cover with chocolate buttercream, smoothing sides and texturing ends.
5. Add bark shards along sides.
6. Scatter pistachio-matcha “moss” around the log and display platter.
7. Arrange mushrooms.
8. Tuck rosemary sprigs.
9. Dust with powdered sugar for snow.