I start by spreading softened butter over one side of every slice of bread. Working on a cutting board keeps my counter clean, since the butter can get a little messy at this stage. I make sure I cover the bread edge to edge so every bite toasts evenly in the skillet later.
Next, I flip half of the buttered slices over so the butter side faces down. On the plain side of these slices, I spread a thin, even layer of cranberry sauce. I keep the rest of the bread slices aside since those will become the tops of my sandwiches.
I arrange the sliced brie and shredded mozzarella right on top of the cranberry sauce, splitting it evenly across all four sandwiches. If I'm using flakey salt, this is when I sprinkle a small pinch over the cheese. Then I top each sandwich with the remaining bread slice, butter side facing up.
I heat my skillet over medium-low heat and place the sandwiches in carefully once it's warm. I cook each side for about 3 to 5 minutes, pressing down gently with my spatula, until the bread turns deep golden brown and the cheese inside is fully melted. I serve these right away while they're still warm and gooey.