Preheat broiler.

Slice tomatoes a half-inch thick; place in a single layer on a plate or tray and season with salt and pepper and let sit until needed. Cook bacon until it is crisp; drain and reserve drippings for another use. 

Spread one side of four pieces of bread with brie. Arrange on baking sheet and carefully broil until cheese is soft; remove. Evenly arrange four slices of bacon over each slice, pressing down into melted cheese. Top each evenly with seasoned tomato and basil. Spread remaining four slices of bread with mayonnaise, and top sandwiches.

Heat a cast-iron skillet over medium-high heat and melt two pats butter. Add two sandwiches to the pan and cook, pressing down with a spatula, until golden brown. Flip and repeat, adding extra butter if needed. Repeat with remaining sandwiches; serve warm.