Place eggs in a large saucepan and fill with cool water. Slowly bring the water to a boil over medium heat. Once boiling, remove the heat and cover the saucepan for 10 minutes.
Drain water and peel eggs shells from the hard-boiled eggs. Cut each hard-boiled egg in half, carefully scoop out yolks and place in a medium bowl. Set the egg whites on a serving plate, cut side up.
To the bowl with egg yolks, add mayonnaise, 3/4 of the chopped bacon (leaving the remainder for garnish), diced avocado, cilantro, lime juice, cayenne pepper, a pinch of kosher salt, and a few turns of black pepper.
Using a fork, mash and stir the yolks until all ingredients are well-combined. It won't be smooth, but you don't want any big chunks of yolk remaining. Season to taste with additional salt, lime juice, and cayenne pepper.
Transfer yolk mixture to a small zip top bag. Seal the bag, then cut off one of the corners to create a small hole for dispensing yolk mixture. Pipe the yolk mixture into the wells of each egg white.
Sprinkle with reserved chopped bacon and light dusting of cayenne pepper.
Preheat oven to 375 degrees F. Place bacon strips on a foil lined cookie sheet. Bake in oven for 15-20 minutes or until bacon reaches desired crispiness.
Butter one side of each piece of bread, smearing the butter to the very edges of the piece of bread.
Place a large flat skillet on burner set to medium heat. Then place two slices of bread, buttered side down, on skillet. Next add half of the brie atop each slice of bread, then add apples on top of the brie. Add second piece of bread, buttered side up, on top of apples.
Grill for approximately 4 minutes, flip sandwiches and grill for another 2 minutes or until bread has turned golden and cheese melted.
Remove from the skillet. Open sandwich on apple side, then add bacon, and serve.