Prep the pan: Heat oven to 350°F (177°C). Line a 10x15-inch jelly roll pan with parchment, leaving a little overhang, and lightly grease the paper. Lay out a clean kitchen towel and dust it generously with cocoa powder.

Make the yolk mixture: In a medium bowl, whisk egg yolks, 1/2 cup of the sugar, and vanilla until pale and slightly thick, about 2 minutes. Sift flour, cocoa, and salt over the bowl and fold until mostly combined. The batter will be thick.

Whip the whites: In a clean large bowl, beat egg whites and cream of tartar on medium-high to soft peaks. Gradually rain in the remaining 1/4 cup sugar and beat to glossy stiff peaks.

Fold and bake: Gently fold one-third of the whites into the yolk mixture to loosen, then fold in the remaining whites in two additions just until no streaks remain. Spread evenly in the prepared pan. Bake 10 to 12 minutes, until the top springs back when touched and edges just pull away.

Roll while warm: Immediately run a knife around edges, invert the cake onto the cocoa-dusted towel, and peel off the parchment. Starting from a short side, roll the warm cake up with the towel. Let it cool seam-side down for 30 minutes.

Make the filling: In a chilled bowl, beat the mascarpone briefly to smooth (if using). Add cold cream, powdered sugar, and vanilla. Whip to medium-stiff peaks, fluffy but spreadable.

Fill and reroll: Carefully unroll the cooled cake. Spread the cream evenly, leaving a 1/2-inch border all around. Reroll gently without the towel. Wrap and chill 20 to 30 minutes to set.

Make ganache: Place chopped chocolate, butter, and a pinch of salt in a bowl. Heat cream to steaming, then pour over chocolate. Let sit 2 minutes, then stir until smooth and glossy. Cool 15 to 25 minutes until thickened to a spreadable consistency.

Trim and frost: Trim one end of the roll on a slight diagonal and place the cutoff along the side like a stump, attaching with a dab of ganache. Spread ganache over the log in swirls and ridges. Drag a fork lightly to create bark lines and knots.

Chill and decorate: Refrigerate 15 to 20 minutes to set. Dust with powdered sugar for snow and add cranberries and rosemary if you like. Slice with a warm serrated knife, wiping between cuts.