Preheat your oven to 180°C (350°F) and line a baking sheet with parchment paper to prevent sticking.
Gently unfold the puff pastry sheet on a lightly floured surface. Using a sharp knife or pastry cutter, cut the pastry into 24 small squares, about 3 inches each, handling the dough carefully to keep it cold and flaky.
Arrange the pastry squares on the prepared baking sheet, spacing them evenly. Place a small piece of brie in the center of each square, pressing down slightly so it stays put during baking.
Top each piece of brie with a teaspoon of cranberries, pressing lightly to embed them into the cheese and pastry.
In a small bowl, whisk together the egg and water to create an egg wash. Using a pastry brush or the back of a spoon, gently brush the edges of each pastry square with the egg wash to help them turn golden and shiny.
Bake the bites in the preheated oven for 12-15 minutes, or until the edges are puffed up and golden brown, the cheese is bubbling, and the cranberries are softened and shiny.
Remove the baking sheet from the oven and transfer the bites to a cooling rack. Let them rest for 5 minutes to allow the cheese to set slightly.
Serve these warm or at room temperature, garnished with a sprig of thyme or a drizzle of honey if desired, and enjoy the delightful contrast of flaky pastry, melty cheese, and tart cranberries.