Thinly slice bread into 8 slices. Heat panini pan.
Spread one side of each bread slice with 1/2 tablespoon butter. Put 4 slices, buttered side down, on a work surface.
Grate chocolate; divide evenly between bread slices. Top with remaining bread, buttered side up. Dust each side of sandiwches with 1/4 teaspoon cinnamon.
Press in heated pan, 3 to 5 minutes total, until filling is melted. Cut into quarters and serve with creme fraiche, for dipping.