Chill the Brie. Remove and discard the rind from the Brie cheese (or leave it on if you prefer). Place the cheese in the freezer for 20 minutes to make it easier to cut. This step is key for clean cuts!

Prep the muffin pan. While the Brie chills, spray a 24-cup mini muffin pan generously with cooking spray. Set aside.

Roll and cut the puff pastry. On a lightly floured surface, roll the puff pastry sheet out into a 10x14-inch rectangle. Cut the sheet lengthwise into 4 even strips, then crosswise into 6 even strips, making 24 squares total.

Press into muffin cups. Separate the pastry squares and gently press each one into a prepared mini muffin cup. The corners will stick up—that's perfect!

Cut the Brie. Remove the chilled Brie from the freezer and cut it into 24 pieces, approximately 3/4-inch in size. Cold Brie cuts cleanly without sticking to the knife.

Assemble the bites. Add 1 teaspoon cranberry sauce to each pastry-lined muffin cup. Press a piece of Brie into the cranberry sauce. Top with 1 teaspoon chopped walnuts. Season each bite with a small pinch of sea salt.

Chill before baking. Transfer the muffin pan to the refrigerator and chill for at least 30 minutes. (You can also cover and chill for up to 3 days at this point!)

Preheat the oven. When ready to bake, preheat your oven to 400°F (200°C).

Bake until golden. Bake the bites in the preheated oven until the pastry is golden brown and puffed, 18 to 20 minutes. The Brie will be melty and the cranberry sauce will be bubbling.

Cool and serve. Let the bites cool in the pan for 5 minutes, then carefully remove with a small offset spatula or butter knife. Serve warm and enjoy! Garnish with fresh rosemary sprigs for a festive touch.