1. Preheat the oven to 350°F (180°C) and line a 10×15-inch baking tray with greased parchment paper.

2. Melt butter and chopped chocolate together in a heatproof bowl over simmering water, then let cool slightly.

3. Whisk granulated sugar, brown sugar, and eggs until frothy, then stir in the vanilla extract and melted chocolate mixture.

4. Sift flour, cocoa powder, baking powder, and salt together and gently fold into the chocolate batter until just combined.

5. Spread the batter evenly in the prepared tray and bake for 12–15 minutes until the edges are set but the center remains slightly gooey.

6. Remove from the oven and let rest for 2–3 minutes, then flip onto a clean towel, peel off parchment, dust with cocoa powder, and roll from a short edge while still warm. Let cool completely rolled up.

7. Heat heavy cream until just simmering, pour over chopped dark chocolate, wait one minute, then stir until smooth. Mix in optional butter for extra shine and cool to spreadable consistency.

8. Unroll the cooled brownie base, spread half the ganache inside leaving a small border, then reroll tightly and transfer to a platter, seam side down.

9. Use an offset spatula to cover the log with remaining ganache, creating rustic bark textures with a fork or spatula.

10. Dust with powdered sugar, decorate with rosemary sprigs or cranberries, then slice and serve at room temperature.