Heat oven to 425ºF (218ºC) with rack set in upper-middle and lower-middle positions. Line two baking sheets with parchment paper.
In a medium mixing bowl, combine 1 pound ground beef, ½ cup breadcrumbs, ¼ cup jam, 1 egg, 2 teaspoons minced fresh rosemary (or 1 teaspoon if using dried), ¾ teaspoon garlic powder, ¾ teaspoon onion powder and 1¼ teaspoons kosher salt (or 1½ teaspoons if using Diamond Crystal) and ½ teaspoon black pepper. Mix until combined.
Scoop meat mixture, by leveled tablespoon, into 36 portions. Roll into balls and arrange on prepared baking sheet. Bake meatballs until cooked through and an instant-read thermometer registers 165ºF (74ºC), 8–10 minutes.
When done baking, let meatballs cool slightly then transfer to a medium mixing bowl and toss with remaining 2 tablespoons jam to coat. Remove and replace parchment paper on baking sheet used to cook meatballs.
Cut 2 sheets of puff pastry into 16 even squares. Arrange evenly on parchment-lined baking sheets.
Cut 12 ounces brie into ¼-inch-thick slices then cut into 1-inch squares. Place a square of brie in center of each pastry square.
Beat remaining 1 egg in a small bowl with a splash of water. Brush edges of pastry with egg wash then sprinkle lightly with kosher salt and rosemary. Bake until puffed, edges are lightly golden and cheese is melted, 10–13 minutes.
Remove sheets from oven and place meatball in center of each pastry. Return sheets to oven and continue baking until pastries are deeply golden brown, 3–5 more minutes.
Optional: brush meatballs with a bit more jam to give them a nice shine. Serve immediately.