Heat a griddle or large skillet over medium-low heat. Alternatively, you can use a panini press if you have one.
Generously spread the Dijon on 4 slices of bread. Top each slice with some of the Cheddar, 3 pieces of bacon, some apple and additional Cheddar. Place a slice of the remaining bread on top of each sandwich and spread with the butter. Place a sandwich on the skillet buttered-side down and butter the remaining side. Cook until the cheese is slightly melted and the bread is browned. Flip the sandwich and continue to cook until the cheese is thoroughly melted and the sandwich is golden brown. Repeat with the remaining sandwiches.