1. Preheat oven to 350°F (175°C). Line a 10×15-inch jelly roll pan with parchment paper and spray with baking spray.

2. Beat egg yolks with 1/2 cup sugar on high speed for 4-5 minutes until thick and pale. Add vanilla extract.

3. In a separate clean bowl, whip egg whites with cream of tartar until foamy. Gradually add remaining 1/4 cup sugar and beat until stiff peaks form.

4. Sift cocoa powder and cake flour over egg yolk mixture. Gently fold in with a rubber spatula.

5. Add one-third of egg whites to chocolate mixture and fold to lighten. Add remaining whites in two additions, folding gently until just combined.

6. Pour batter into prepared pan and spread evenly. Bake for 12-15 minutes until cake springs back when touched.

7. Immediately invert hot cake onto a powdered sugar-dusted towel. Peel off parchment paper and roll cake with towel from short end. Cool completely for 45 minutes to 1 hour.

8. Melt chopped chocolate and allow to cool to room temperature. Beat butter for 3-4 minutes until fluffy and pale.

9. Add powdered sugar and cocoa powder to butter, mixing on low speed. Add cooled melted chocolate, vanilla extract, and heavy cream. Beat on medium-high for 2-3 minutes until fluffy.

10. Carefully unroll the cooled cake. Spread one-third of buttercream evenly over surface, leaving 1/2-inch border.

11. Re-roll cake tightly without the towel. Transfer to serving platter seam-side down.

12. Cut a 2-inch diagonal slice from one end and attach to side of log with buttercream to create a branch effect.

13. Frost entire log with remaining buttercream. Use a fork to create bark texture by dragging it lengthwise through frosting.

14. Dust with powdered sugar for snow effect. Decorate with rosemary sprigs and fresh cranberries. Refrigerate for 1 hour before serving.