Preheat the oven to 425 degrees F. Set a rack on a rimmed baking sheet.
Lay the pancetta slices on the rack on the baking sheet and bake until crisp, 15 to 18 minutes.
Meanwhile, set your toaster to a low setting and toast the brioche slices, (they should be slightly dry but not brown).
Lay 4 slices lightly toasted brioche on a work surface and cover the slices with half the cheese, followed by a slice of tomato. Season each sandwich with a pinch of salt and a healthy grind of black pepper. Top each with 2 pancetta rounds and the remaining cheese. Lay the remaining brioche slices on top and press down lightly. Butter the top of each sandwich with 1/2 tablespoon butter.
Heat a large griddle pan over medium heat.
Melt the remaining tablespoon butter on the griddle pan and add the sandwiches, buttered side up. Cook until the bread is golden brown and the cheese is beginning to melt. Flip the sandwiches, press lightly, and continue cooking until golden and toasty. Transfer to a cutting board and slice the sandwiches with a serrated knife, on the diagonal. Arrange on a serving platter and serve immediately.