Lightly spread bread with butter. Cover 6 bread slices with prosciutto, cutting to fit. Crumble cheese or dab overtop, then spread evenly as best you can. Scatter with arugula. Top with remaining bread slices, butter-side down. Gently press down. Trim and discard crusts from bread, if you like. If making ahead, wrap and refrigerate up to a day. Before serving, cut each sandwich into 4 triangles.