Preheat the oven to 375°F (190°C). Lightly flour a work surface.
Roll the thawed puff pastry into an approximately 12-by-12-inch square, slightly enlarging the sheet if needed.
Cut the pastry into 25 equal squares and gently press each square into the cups of a 24-cup mini muffin tin to form small cups.
Bake the pastry cups for 10 minutes, until puffed. Using the round end of a wooden spoon, press the center of each cup down, then bake 3–5 minutes more until the corners are just beginning to brown.
Place one 1/2-inch cube of brie into the center of each pastry cup. Top each with about 1/4 teaspoon of fruit preserves and a sprinkle of chopped pistachios.
Return the filled cups to the oven and bake 3–5 minutes more, until the cheese is melted and the pastry is light golden brown.
Remove from the oven and let cool in the tin for about 5 minutes, then carefully lift the puffs out and serve warm.