Preheat oven to 350°F (177°C). Lightly grease a mini muffin tin.
On a lightly floured surface, cut into 24 equal squares (about 2x2 inches each).
Press each pastry square into a mini muffin cup, letting the corners stick up.
Place a cube of brie in each pastry cup and spoon in about 1 teaspoon of jelly.
Bake for 15-20 minutes, or until pastry is golden brown and cheese is melted.
Sprinkle chopped pistachios over the jelly.
Let cool for 5 minutes before serving.
Notes
If you don't have a mini muffin tin: You can cut your puff pastry into larger squares and use bigger chunks of brie to bake in a regular muffin pan.Keep the puff pastry cold! This is so that it bakes properly! It does need to thaw if it's frozen, but it's important to the baking process for it to be kept cold.Be careful! Try not to overfill the muffin cups or they'll spill over the sides of the pastry when baking. The sugars in the jam or sauce you use will caramelize and burn quickly if they escape to the hot pan.