In a small bowl, stir together the mayonnaise, mustard and horseradish. In another small bowl, stir together the honey and truffle oil. Smear one side of the bread slices with mayonnaise mixture. Top 3 slices of bread with cheese. Top with remaining bread slices, mayonnaise side down.
Heat a large non-stick skillet over medium heat. Brush the top bread slices with half the melted butter. Place the sandwiches into the skillet buttered side down. Brush the top bread slices with the remaining butter. Cook until the bread is toasted and cheese melts, about 3 minutes per side.
Cut each sandwich into 8 triangles and place on a serving dish. Drizzle some of the truffle honey over the triangles and serve immediately with the remaining honey on the side for dipping.