Preheat oven to 350°. Line a jelly roll pan with parchment paper and spray with cooking spray. In a medium bowl mix together flour, cocoa powder, and salt.
In a large bowl beat egg yolks until thick. Slowly add ½ cup sugar and beat until pale then beat in flour mixture.
In another large bowl beat egg whites until soft peaks form. Add remaining ¼ cup sugar gradually and continue to beat until stiff peaks form. Gently fold egg whites into batter in two batches.
Pour batter into prepared pan and spread into an even layer. Bake until top springs back when lightly pressed, about 12 minutes.
Dust a clean kitchen towel with powdered sugar and invert warm cake onto towel. Peel off parchment paper.
Starting at the short end, use the towel to tightly roll cake into a log. Let cool completely.
Make filling: if using gelatin, place 2 tablespoons cold water in a shallow microwave-safe bowl and sprinkle gelatin in an even layer on top. Let bloom for 5 to 10 minutes, then microwave for 10 seconds until gelatin is runny—do not overheat!
In a large bowl, beat together cream, powdered sugar, vanilla extract, and a pinch of salt, until medium peaks form. If using gelatin, strain and stream into whipped cream at this point while continuing to beat the cream. Refrigerate until ready to use.
When cake is cool, unroll and spread filling evenly over cake. Roll cake back into a log, using the towel to help create a tight roll. Place seam side down on a baking sheet and refrigerate until well chilled, approximately 1 hour.
To make frosting: In a large bowl beat butter until smooth. Add powdered sugar and cocoa powder and beat until no lumps remain then beat in vanilla, heavy cream, and salt.
When ready to serve, trim ends and frost cake with chocolate buttercream. Dust lightly with powdered sugar and top with chocolate curls. Place berries and rosemary on log to create mistletoe. You can also add leftover clumps of sugar to make a “snow” effect.
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