Preheat your oven to 350°F (175°C). Line a rimmed baking sheet with parchment paper.
Using a sharp serrated bread knife, slice the sourdough loaf in a crosshatch pattern, making cuts about 1 inch apart. Be careful not to cut all the way through the bottom; leave about ½ inch intact.
Gently pull apart the bread sections and tuck the cubed Brie cheese into the openings, distributing it evenly throughout the loaf.
Spoon the whole-berry cranberry sauce into the bread slits, making sure to spread it between the pieces of Brie.
In a small bowl, combine the melted butter, honey, chopped rosemary, and garlic powder. Stir until well mixed.
Brush about two-thirds of the honey-herb butter mixture over the entire loaf, making sure some gets into the bread cuts.
Loosely cover the bread with aluminum foil and bake for 15 minutes to help the Brie melt without over-browning the crust.
Remove the foil, brush with the remaining butter mixture, and bake uncovered for another 5–10 minutes, or until the Brie is melted and the bread is golden brown.
Remove from the oven, drizzle with additional honey, and sprinkle with flaky sea salt if desired.
Transfer to a serving board or platter and serve immediately while the cheese is warm and gooey.