Make sure the 10 sheets phyllo dough are completely thawed according to the package directions. Keep the sheets covered with a clean, slightly damp kitchen towel while working so they do not dry out and crack.

Preheat the oven to 375°F (190°C). Lightly grease a 9-inch round cake pan or pie dish with some of the melted butter.

Place one sheet of phyllo in the pan, allowing the edges to extend slightly over the rim. Brush lightly with melted butter. Add another sheet, rotate it slightly, and brush with butter. Continue with 5 sheets total, brushing each layer lightly with butter.

Arrange the 8 ounces Brie over the phyllo, leaving a small border around the edge. Spoon the 3/4 cup cranberry sauce evenly over the Brie. Sprinkle with the 1 tablespoon chopped rosemary and 1/4 teaspoon black pepper.

Place another sheet of phyllo over the filling and brush lightly with butter. Repeat with the remaining 4 sheets, rotating each slightly as you layer them. Tuck the edges gently around the filling or fold the overhanging pastry inward to create a rustic layered top.

Brush the top generously but evenly with the remaining melted butter. If using, sprinkle the 1 tablespoon sesame seeds over the surface. Use a sharp knife to make a few shallow cuts through the top layers of phyllo. Do not cut deeply into the filling.

Bake for 25–30 minutes, checking around the 25-minute mark. The pastry should be crisp, puffed, and deep golden brown. If the top is browning too quickly while the center still needs time, loosely cover it with foil and continue baking.

Remove the pastry from the oven and let it rest for 10 minutes. The Brie should be soft and creamy, while the cranberry filling should be warm and thick rather than runny.

Cut the pastry into 10 wedges with a sharp knife. Serve warm or slightly cooled.