Roast Beef and Horseradish Finger Sandwiches

Combine mayonnaise and horseradish. Spread 5 bread slices with horseradish-mayonnaise mixture. Top with roast beef slices. Spread butter thinly over remaining bread slices and top beef with the buttered side down. Using a serrated knife, cut off crusts. Cut each sandwich in half, then cut each half into 3 even fingers. Serve chilled.