Preheat your oven to 200°C (390°F). Line a baking sheet with parchment paper to keep things tidy.
Gently unfold the puff pastry sheet and lightly flour your work surface. Roll out the pastry to about 0.5 cm thickness, creating an even square roughly 25x25 cm.
Using a sharp knife or pizza cutter, cut the pastry into 24 small squares, about 2.5 cm wide. Transfer these squares onto your prepared baking sheet, spacing them evenly.
Chill the pastry squares in the fridge for 10 minutes. Meanwhile, cut the brie into small pieces and set aside.
Remove the baking sheet from the fridge and bake the pastry squares for 8–10 minutes, until they start to turn golden and puff up slightly.
Gently press down the center of each baked pastry square with the back of a teaspoon to create a small indentation. Carefully fill each with a small piece of brie and a few cranberries.
Return the tray to the oven and bake for another 5–7 minutes, until the brie is melted and bubbly, and the cranberries are soft.
Remove the bites from the oven. Brush the pastry edges lightly with the beaten egg to give them a shiny, golden finish. If desired, sprinkle with a pinch of flaky sea salt and chopped fresh herbs.
Let the cranberry brie bites rest for 2 minutes to allow the cheese to set slightly. Serve warm or at room temperature for the best flavor and texture.