Spread butter over one side of each bread slice.
Flip half of the bread slices over, so they’re butter-side down. Spread each of these slices with a layer of cranberry sauce. Reserve the remaining slices for the top of your sandwiches.
Layer brie and mozzarella on top of the cranberry sauce, dividing cheeses evenly between each sandwich, then add a pinch of flakey salt (optional), and top each sandwich with a remaining slice of bread (butter-side up).
Heat a large skillet over medium-low heat. Carefully place sandwiches in the hot skillet and cook for 3-5 minutes per side until bread is golden brown and cheese has melted. Serve immediately.
