Melt 3 tbsp butter in a large skillet over medium-high heat. Once melted, toss mushrooms in butter to coat, then season with salt and pepper. Allow the mushrooms to cook for about 2-3 minutes per side, or until they are very slightly brown.
Remove buttered mushrooms from skillet and place bottom-side up in a large baking dish.
Reduce heat under the skillet to medium-low, add the remaining 1 tbsp of butter to the pan and allow to melt.
Once melted, add the capers and garlic. Cook for 3-5 minutes, or until the garlic begins to soften. Stir frequently to prevent the garlic from sticking/burning.
Once garlic has begun to soften, add the panko break crumbs. Season with salt and pepper. Stir until the crumbs are completely coated in butter, then remove the pan from heat. Stir in parsley (if using).
Add a piece of brie to each mushroom, pushing down gently.
Spoon about 1 tsp of the panko mixture on top of each mushroom.
Place mushrooms in the oven and bake for about 20-30 minutes, or until the panko crumbs are golden and the mushrooms become darker in colour.
Remove the mushroom caps from the oven, and allow to cool slightly before serving.
You do not have to remove the rind from the Brie before adding it to the mushrooms. I tried this recipe with and without the rind, and couldn't taste a difference.
If your skillet isn't large enough to hold all the mushrooms at once, you can melt half the amount of butter at a time and coat the mushrooms in batches.
If your baking dish isn't large enough to hold all the mushrooms at once, or if they are very crowded in the dish, you can cook this in 2 baking dishes. Just rotate the dishes halfway through cooking so everything cooks evenly.
Please note that the nutritional information for this recipe is only an estimate, and has been calculated using a plugin.