In a medium bowl, mix together cinnamon and sugar.
Spread butter on one side of all four slices of bread. And sprinkle each with a generous amount of cinnamon/sugar mixture.
Toss sliced apples in remaining cinnamon/sugar mixture.
Place bread butter-side-down on grill pan over medium-low heat and top each slice with brie cheese. Layer two of the slices with cinnamon sugar apples and then sandwich brie-only slices of bread with apple slices.
Press down with a spatula and let grill for about 2 minutes, until bread begins turning golden. Flip sandwiches over and cook for another 2 minutes, until bread turns golden and cheese gets melty. Slice and serve!